Evaluating radio frequency (rf) exposure in real time

ABSTRACT

In certain aspects, a method implemented in a wireless device includes determining a specific absorption rate (SAR) distribution for a first wireless communication technology, determining a power density (PD) distribution for a second wireless communication technology, and combining the SAR distribution and the PD distribution to generate a combined RF exposure distribution. The method also includes determining at least one first maximum allowable power level and at least one second maximum allowable power level for a future time slot based on the combined RF exposure distribution, setting at least one transmission power limit for a first transmitter in the future time slot based on the at least one first maximum allowable power level, and setting at least one transmission power limit for a second transmitter in the future time slot based on the at least one second maximum allowable power level.

RELATED APPLICATIONS

This application claims priority to International Application No.PCT/US2019/040556 filed on Jul. 3, 2019, which claims priority to U.S.application Ser. No. 16/460,894 filed on Jul. 2, 2019, U.S. ProvisionalApplication No. 62/694,405 filed on Jul. 5, 2018, and U.S. ProvisionalApplication No. 62/696,687 filed on Jul. 11, 2018, the entirespecifications of which are incorporated herein by reference. Further,this application claims benefit to European Application No. 19745440.8filed on Dec. 22, 2020, Indian Application No. 20204705630 filed on Dec.24, 2020 and Japanese Application No. Unknown filed on Dec. 25, 2020,all of which claim priority to International Application No.PCT/US2019/040556 filed on Jul. 3, 2019.

BACKGROUND Field

Aspects of the present disclosure relate generally to wireless devices,and more particularly, to assessing radio frequency (RF) exposure from awireless device.

Background

Modern wireless devices (e.g., cellular phones) are generally requiredto limit a user's exposure to radio frequency (RF) radiation accordingto RF exposure limits set by domestic and international regulators. Toensure that a wireless device complies with an RF exposure limit,techniques have been developed to enable the wireless device to assessRF exposure from the wireless device in real time and adjust thetransmission power of the wireless device accordingly to comply with theRF exposure limit.

SUMMARY

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of such embodiments. Thissummary is not an extensive overview of all contemplated embodiments,and is intended to neither identify key or critical elements of allembodiments nor delineate the scope of any or all embodiments. Its solepurpose is to present some concepts of one or more embodiments in asimplified form as a prelude to the more detailed description that ispresented later.

A first aspect relates to a wireless device. The wireless deviceincludes a first transmitter configured to transmit first signalsaccording to a first wireless communication technology, a secondtransmitter configured to transmit second signals according to a secondwireless communication technology, and a processor coupled to the firstand second transmitters. The processor is configured to determine aspecific absorption rate (SAR) distribution for the first wirelesscommunication technology, determine a power density (PD) distributionfor the second wireless communication technology, combine the SARdistribution and the PD distribution to generate a combined RF exposuredistribution, determine at least one first maximum allowable power leveland at least one second maximum allowable power level for a future timeslot based on the combined RF exposure distribution, set at least onetransmission power limit for the first transmitter in the future timeslot based on the at least one first maximum allowable power level, andset at least one transmission power limit for the second transmitter inthe future time slot based on the at least one second maximum allowablepower level.

A second aspect relates to a method implemented in a wireless device.The method includes determining a specific absorption rate (SAR)distribution for a first wireless communication technology, determininga power density (PD) distribution for a second wireless communicationtechnology, and combining the SAR distribution and the PD distributionto generate a combined RF exposure distribution. The method alsoincludes determining at least one first maximum allowable power leveland at least one second maximum allowable power level for a future timeslot based on the combined RF exposure distribution, setting at leastone transmission power limit for a first transmitter in the future timeslot based on the at least one first maximum allowable power level, andsetting at least one transmission power limit for a second transmitterin the future time slot based on the at least one second maximumallowable power level.

A third aspect relates to a computer readable medium. The computerreadable medium includes instructions stored thereon for determining aspecific absorption rate (SAR) distribution for a first wirelesscommunication technology, determining a power density (PD) distributionfor a second wireless communication technology, and combining the SARdistribution and the PD distribution to generate a combined RF exposuredistribution. The computer readable medium also includes instructionsstored thereon for determining at least one first maximum allowablepower level and at least one second maximum allowable power level for afuture time slot based on the combined RF exposure distribution, settingat least one transmission power limit for a first transmitter in thefuture time slot based on the at least one first maximum allowable powerlevel, and setting at least one transmission power limit for a secondtransmitter in the future time slot based on the at least one secondmaximum allowable power level.

A fourth aspect relates to a wireless device. The wireless deviceincludes a transmitter, and a processor coupled to the transmitter. Theprocessor is configured to determine a specific absorption rate (SAR)distribution for a first wireless communication technology, determine apower density (PD) distribution for a second wireless communicationtechnology, combine the SAR distribution and the PD distribution togenerate a combined RF exposure distribution, and determine a maximumallowable time-average power level for a future time slot based on thecombined RF exposure distribution, wherein the future time slotcomprises multiple sub-time slots. The processor is also configured todetermine a maximum allowable power level for each of the multiplesub-time slots based on the maximum allowable time-average power levelfor the future time slot, and set a transmission power limit for thetransmitter in each of the sub-time slots based on the respectivemaximum allowable power level.

A fifth aspect relates to a wireless device. The wireless deviceincludes a transmitter, and a processor coupled to the transmitter. Theprocessor is configured to determine a specific absorption rate (SAR)distribution for a first wireless communication technology, determine apower density (PD) distribution for a second wireless communicationtechnology, combine the SAR distribution and the PD distribution togenerate a combined RF exposure distribution, and determine a PD limitfor a future time slot based on the combined RF exposure distribution,wherein the future time slot comprises multiple sub-time slots. Theprocessor is also configured to determine a maximum allowable powerlevel for each of the multiple sub-time slots based on the PD limit forthe future time slot, and set a transmission power limit for thetransmitter in each of the sub-time slots based on the respectivemaximum allowable power level.

A sixth aspect relates to a wireless device. The wireless deviceincludes a transmitter, and a processor coupled to the transmitter. Theprocessor is configured to determine a maximum allowable time-averagepower level for a future time slot, determine a maximum allowabletransmission duty cycle for the future time slot based on the determinedmaximum allowable time-average power level and a maximum allowable powerlevel, and set a transmission duty cycle limit for the transmitter inthe future time slot based on the maximum allowable transmission dutycycle.

A seventh aspect relates to a wireless device. The wireless deviceincludes a transmitter, and a processor coupled to the transmitter. Theprocessor is configured to determine a maximum allowable time-averagepower level for a future time slot, determine a maximum allowable peakpower level for the future time slot based on the determined maximumallowable time-average power level and a maximum allowable duty cycle,and set a peak power limit for the transmitter in the future time slotbased on the maximum allowable peak power level.

To the accomplishment of the foregoing and related ends, the one or moreembodiments include the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative aspects ofthe one or more embodiments. These aspects are indicative, however, ofbut a few of the various ways in which the principles of variousembodiments may be employed and the described embodiments are intendedto include all such aspects and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a wireless device in which aspects of thepresent disclosure may be implemented.

FIG. 2 shows an example of a normalized specific absorption rate (SAR)distribution combined with a normalized power density (PD) distributionaccording to certain aspects of the present disclosure.

FIG. 3 is a flowchart illustrating an exemplary method for determiningtransmission power levels that comply with RF exposure limits forsimultaneous transmissions using multiple wireless communicationtechnologies according to certain aspects of the present disclosure.

FIG. 4 is a flowchart illustrating an exemplary method for determiningtransmission power levels that comply with a PD limit according tocertain aspects of the present disclosure.

FIG. 5 shows an example of a time-averaged SAR distribution according tocertain aspects of the present disclosure.

FIG. 6 is a flowchart illustrating an exemplary method for determiningtransmission power levels for a future time slot in compliance with atime-average SAR limit according to certain aspects of the presentdisclosure.

FIG. 7 shows an example of a time-averaged PD distribution according tocertain aspects of the present disclosure.

FIG. 8 is a flowchart illustrating an exemplary method for determiningtransmission power levels that comply with a time-average PD limitaccording to certain aspects of the present disclosure.

FIG. 9 shows an example of a time-averaged SAR distribution combinedwith a time-averaged PD distribution according to certain aspects of thepresent disclosure.

FIG. 10 is a flowchart illustrating an exemplary method for determiningtransmission power levels that comply with time-average RF exposurelimits according to certain aspects of the present disclosure.

FIG. 11 shows an example in which a time-averaged PD distribution isdetermined using multiple time-averaging windows for different frequencybands according to certain aspects of the present disclosure.

FIG. 12 shows an example in which a time-averaged PD distribution isdetermined for simultaneous transmissions at different frequency bandsaccording to certain aspects of the present disclosure.

FIG. 13 shows an example of a time-averaged SAR distribution combinedwith a PD distribution according to certain aspects of the presentdisclosure.

FIG. 14 is a flowchart showing an example of a method implemented in awireless device according to certain aspects of the present disclosure.

FIG. 15 shows an example in which maximum allowable power levels aredetermined at different rates for different technologies according tocertain aspects of the present disclosure.

FIG. 16 shows an example of PD over time for an inner loop according tocertain aspects of the present disclosure.

FIG. 17 shows an example of a used portion of a PD allocation, and aportion of the PD allocation reserved for future transmissions accordingto certain aspects of the present disclosure.

FIG. 18 shows an exemplary table for converting an allowed PD into amaximum allowable power level according to certain aspects of thepresent disclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

FIG. 1 shows an example of a wireless device 100 in which aspects of thepresent disclosure described herein may be implemented. The wirelessdevice 100 may comprise a mobile wireless device (e.g., a cellularphone), a laptop, a wireless access point, or some other wirelessdevice.

The wireless device 100 includes a processor 110, and a memory 115coupled to the processor 110. The memory 115 may store instructionsthat, when executed by the processor 110, cause the processor 110 toperform one or more of the operations described herein. The processor110 may be implemented with a general-purpose processor, a digitalsignal processor (DSP), a baseband modem, an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device (PLD), discrete gate logic, discretehardware components, or any combination thereof configured to performone or more of the operations described herein.

The wireless device 100 also includes a first transmitter 120, a firstplurality of antennas 122-1 to 122-N coupled to the first transmitter120, and a first bus 140 coupled between the first transmitter 120 andthe processor 110. In certain aspects, the first transmitter 120 isconfigured to transmit signals via one or more of the first plurality ofantennas 122-1 to 122-N using one or more wireless communicationtechnologies, including, but not limited to, a third generation (3G)technology (e.g., CDMA), a fourth generation (4G) technology (also knownas Long Term Evolution (LTE)), a fifth generation (5G) technology, oneor more technologies based on one or more IEEE 802.11 protocols (e.g.,IEEE 802.11ac, IEEE 802.11n, IEEE 802.11ad, IEEE 802.11ax, IEEE802.11ay, etc.), and/or one or more other technologies. In some aspects,the first transmitter 120 may be configured to transmit signals viamultiple antennas 122-1 to 122-N using multiple-input-multiple-output(MIMO) transmission to increase the capacity of a radio link between thewireless device 100 and another wireless device (not shown). In someaspects, the first transmitter 120 may be configured to transmit signalsvia multiple antennas 122-1 to 122-N using beamforming to directtransmissions toward another wireless device (not shown). In theseaspects, the transmissions may be electrically steered by adjusting therelative phases and/or amplitudes of the transmit signals for thedifferent antennas 122-1 to 122-N.

The processor 110 interfaces with the first transmitter 120 via thefirst bus 140. The first bus 140 may include one or more signal linesbetween the processor 110 and the first transmitter 120. To transmitdata, the processor 110 may process the data into one or more signals(e.g., baseband signals or intermediate-frequency signals). Theprocessing performed by the processor 110 may include coding the dataand modulating the coded data (e.g., using any one of a variety ofdifferent modulation schemes, including BPSK, QPSK, QAM, etc.). Theprocessor 110 may output the one or more signals to the firsttransmitter 120 via the first bus 140. The first transmitter 120 maythen process the one or more signals from the processor 110 into one ormore RF signals for transmission via one or more of the antennas 122-1to 122-N. The processing performed by the first transmitter 120 mayinclude frequency up-conversion, power amplification, etc.

In certain aspects, the processor 110 may adjust the transmission powerfor one or more of the antennas 122-1 to 122-N. For example, the firsttransmitter 120 may include multiple amplifiers (not shown) where eachof the amplifiers is coupled to a respective one of the antennas. Foreach amplifier, the processor 110 may output a respective control signalto the amplifier via the first bus 140 to control the gain of theamplifier. In this example, the processor 110 may adjust thetransmission power for an antenna by adjusting the gain of therespective amplifier accordingly. In another example, the processor 110may output one or more signals to the first transmitter 120 where eachof the one or more signals corresponds to a respective one of theantennas 122-1 to 122-N. In this example, the processor 110 may adjustthe transmission power for an antenna by adjusting the amplitude of therespective signal accordingly. It is to be appreciated that the presentdisclosure is not limited to the above examples, and that the processor110 may employ other techniques to adjust transmission power.

In certain aspects, the processor 110 may adjust transmission power forone or more of the antennas 122-1 to 122-N using an open power controlloop and/or a closed power control loop. For the example of an openpower control loop, the wireless device 100 may receive a pilot signalfrom another wireless device (not shown) via a receiver (not shown). Inthis example, the processor 110 estimates channel conditions between thewireless device 100 and the other wireless device based on the receivedpilot signal, and adjusts the transmission power for one or more of theantennas 122-1 to 122-N based on the estimated channel conditions. Forthe example of a closed power control loop, the wireless device 100receives a feedback signal from the other wireless device via a receiver(not shown), in which the feedback signal indicates channel conditionsbetween the wireless device 100 and the other wireless device. In thisexample, the processor 110 adjusts the transmission power for one ormore of the antennas 122-1 to 122-N based on the indicated channelconditions.

The processor 110 may also adjust transmission power for one or more ofthe antennas 122-1 to 122-N based on data rate. For example, theprocesser 110 may increase (boost) the transmission power to transmit ashort data burst.

Further, the processor 110 may adjust transmission power for one or moreof the antennas 122-1 to 122-N to keep RF exposure from the wirelessdevice 100 within an RF exposure limit set by a regulator (e.g., FCC),as discussed further below. In this case, the transmission power isconstrained by the RF exposure limit.

The wireless device 100 also includes a second transmitter 130, a secondplurality of antennas 132-1 to 132-M coupled to the second transmitter130, and a second bus 150 coupled between the second transmitter 130 andthe processor 110. In certain aspects, the second transmitter 130 isconfigured to transmit signals via one or more of the second pluralityof antennas 132-1 to 132-M using one or more wireless communicationtechnologies, including, but not limited to, a 3G technology, a 4Gtechnology, a 5G technology, one or more technologies based on one ormore IEEE 802.11 protocols (e.g., IEEE 802.11ac, IEEE 802.11n, IEEE802.11ad, IEEE 802.11ax, IEEE 802.11ay, etc.), and/or one or more othertechnologies. The second transmitter 130 may transmit signals viamultiple antennas 132-1 to 132-M using MIMO transmission, beamforming,and/or other method. In certain aspects, the first transmitter 120 andthe second transmitter 130 may simultaneously transmit signals usingdifferent wireless communication technologies, as discussed furtherbelow.

The processor 110 interfaces with the second transmitter 130 via thesecond bus 150, which may include one or more signal lines between theprocessor 110 and the second transmitter 130. To transmit data, theprocessor 110 may process the data into one or more signals (e.g.,baseband signals or intermediate-frequency signals). The processingperformed by the processor 110 may include coding the data andmodulating the coded data (e.g., using any one of a variety of differentmodulation schemes, including BPSK, QPSK, QAM, etc.). The processor 110may output the one or more signals to the second transmitter 130 via thesecond bus 150. The second transmitter 130 may then process the one ormore signals from the processor 110 into one or more RF signals fortransmission via one or more of the antennas 132-1 to 132-M. Theprocessing performed by the second transmitter 130 may include frequencyup-conversion, power amplification, etc.

The processor 110 may adjust the transmission power for one or more ofthe antennas 132-1 to 132-M. For example, the second transmitter 130 mayinclude multiple amplifiers (not shown) where each of the amplifiers iscoupled to a respective one of the antennas 132-1 to 132-M. For eachamplifier, the processor 110 may output a respective control signal tothe amplifier via the second bus 150 to control the gain of theamplifier. In this example, the processor 110 may adjust thetransmission power for an antenna by adjusting the gain of therespective amplifier accordingly. In another example, the processor 110may output one or more signals to the second transmitter 130 where eachof the one or more signals corresponds to a respective one of theantennas 132-1 to 132-M. In this example, the processor 110 may adjustthe transmission power for an antenna by adjusting the amplitude of therespective signal accordingly. It is to be appreciated that the presentdisclosure is not limited to the above examples, and that the processor110 may employ other techniques to adjust transmission power.

The processor 110 may adjust transmission power for one or more of theantennas 132-1 to 132-M using an open power control loop and/or a closedpower control loop, as discussed above. The processor 110 may alsoadjust transmission power for one or more of the antennas 132-1 to 132-Mto keep RF exposure from the wireless device 100 within an RF exposurelimit set by a regulator, as discussed further below.

It is to be appreciated that the wireless device 100 may include one ormore additional transmitters in addition to the first and secondtransmitters 120 and 130 shown in FIG. 1. Although the first and secondtransmitters 120 and 130 are coupled to separate sets of antennas in theexample shown in FIG. 1, it is to be appreciated that the first andsecond transmitters 120 and 130 may share one or more antennas. Also, insome implementations, the first transmitter 120 may transmit on only oneantenna and/or the second transmitter 130 may transmit on only oneantenna.

Modern wireless devices (e.g., cellular phones) are generally requiredto limit a user's exposure to radio frequency (RF) radiation accordingto exposure limits set by domestic and international regulators. RFexposure may be expressed in terms of a specific absorption rate (SAR),which measures energy absorption by human tissue per unit mass and mayhave units of watts per kilogram (W/kg). RF exposure may also beexpressed in terms of power density (PD), which measures energyabsorption per unit area and may have units of mW/cm2.

SAR may be used to assess RF exposure for transmission frequencies lessthan 10 GHz, which cover wireless communication technologies such as 3G(e.g., CDMA), 4G, IEEE 802.11ac, etc. PD may be used to assess RFexposure for transmission frequencies higher than 10 GHz, which coverwireless communication technologies such as IEEE 802.11ad, 5G, etc.Thus, different metrics may be used to assess RF exposure for differentwireless communication technologies.

The wireless device 100 may simultaneously transmit signals usingmultiple wireless communication technologies. For example, the wirelessdevice 100 may simultaneously transmit signals using a first wirelesscommunication technology operating below 10 GHz (e.g., 3G, 4G, etc.) anda second wireless communication technology operating above 10 GHz (e.g.,5G, IEEE 802.11ad). Since the wireless device 100 simultaneouslytransmits signals using the first and second technologies, the user ofthe device is exposed to RF radiation from transmissions using bothtechnologies. Accordingly, techniques are needed for determining RFexposure compliance for cases where the wireless device 100simultaneously transmits signals using multiple wireless communicationtechnologies.

Aspects of the present disclosure enable the wireless device 100 toassess RF exposure in real time for cases where the wireless device 100simultaneously transmits signals using multiple wireless communicationtechnologies, as discussed further below.

In certain aspects, the wireless device 100 may simultaneously transmitsignals using the first wireless communication technology (e.g., 3G, 4G,IEEE 802.11ac, etc.) in which RF exposure is measured in terms of SAR,and the second wireless communication technology (e.g., 5G, IEEE802.11ad, etc.) in which RF exposure is measured in terms of PD. Inthese aspects, the first transmitter 120 may transmit first signalsaccording to the first wireless communication technology, and the secondtransmitter 130 may transmit second signals according to the secondwireless communication technology. When the wireless device 100simultaneously transmits the first and second signals using the firstand second technologies, respectively, the processor 110 may assess thecombined RF exposure from the first and second technologies to ensurecompliance with RF exposure limits, as discussed further below.

To assess RF exposure from transmissions using the first technology(e.g., 3G, 4G, IEEE 802.11ac, etc.), the wireless device 100 may includemultiple SAR distributions for the first technology stored in the memory115. Each of the SAR distributions may correspond to a respective one ofmultiple transmit scenarios supported by the wireless device 100 for thefirst technology. The transmit scenarios may correspond to variouscombinations of antennas 122-1 to 122-N, frequency bands, channelsand/or body positions, as discussed further below.

The SAR distribution (also referred to as a SAR map) for each transmitscenario may be generated based on measurements (e.g., E-fieldmeasurements) performed in a test laboratory using a model of a humanbody. After the SAR distributions are generated, the SAR distributionsare stored in the memory 115 to enable the processor 110 to assess RFexposure in real time, as discussed further below. Each SAR distributionincludes a set of SAR values, where each SAR value may correspond to adifferent location (e.g., on the model of the human body). Each SARvalue may comprise a SAR value averaged over a mass of 1 g or 10 g atthe respective location.

The SAR values in each SAR distribution correspond to a particulartransmission power level (e.g., the transmission power level at whichthe SAR values were measured in the test laboratory). Since SAR scaleswith transmission power level, the processor 110 may scale a SARdistribution for any transmission power level by multiplying each SARvalue in the SAR distribution by the following transmission powerscaler:

$\begin{matrix}\frac{{Tx}_{c}}{{Tx}_{SAR}} & (1)\end{matrix}$

where Tx_(c) is a current transmission power level for the respectivetransmit scenario, and Tx_(SAR) is the transmission power levelcorresponding to the SAR values in the stored SAR distribution (e.g.,the transmission power level at which the SAR values were measured inthe test laboratory).

As discussed above, the wireless device 100 may support multipletransmit scenarios for the first technology. In certain aspects, thetransmit scenarios may be specified by a set of parameters. The set ofparameters may include one or more of the following: an antennaparameter indicating one or more antennas used for transmission (i.e.,active antennas), a frequency band parameter indicating one or morefrequency bands used for transmission (i.e., active frequency bands), achannel parameter indicating one or more channels used for transmission(i.e., active channels), a body position parameter indicating thelocation of the wireless device 100 relative to the user's body location(head, trunk, away from the body, etc.), and/or other parameters. Incases where the wireless device 100 supports a large number of transmitscenarios, it may be very time-consuming and expensive to performmeasurements for each transmit scenario in a test setting (e.g., testlaboratory). To reduce test time, measurements may be performed for asubset of the transmit scenarios to generate SAR distributions for thesubset of transmit scenarios. In this example, the SAR distribution foreach of the remaining transmit scenarios may be generated by combiningtwo or more of the SAR distributions for the subset of transmitscenarios, as discussed further below.

For example, SAR measurements may be performed for each one of theantennas 122-1 to 122-N to generate a SAR distribution for each one ofthe antennas 122-1 to 122-N. In this example, a SAR distribution for atransmit scenario in which two or more of the antennas 122-1 to 122-Nare active may be generated by combining the SAR distributions for thetwo or more active antennas.

In another example, SAR measurements may be performed for each one ofmultiple frequency bands to generate a SAR distribution for each one ofthe multiple frequency bands. In this example, a SAR distribution for atransmit scenario in which two or more frequency bands are active may begenerated by combining the SAR distributions for the two or more activefrequency bands.

In certain aspects, a SAR distribution may be normalized with respect toan SAR limit by dividing each SAR value in the SAR distribution by theSAR limit. In this case, a normalized SAR value exceeds the SAR limitwhen the normalized SAR value is greater than one, and is below the SARlimit when the normalized SAR value is less than one. In these aspects,each of the SAR distributions stored in the memory 115 may be normalizedwith respect to a SAR limit.

In certain aspects, the normalized SAR distribution for a transmitscenario may be generated by combining two or more normalized SARdistributions. For example, a normalized SAR distribution for a transmitscenario in which two or more antennas are active may be generated bycombining the normalized SAR distributions for the two or more activeantennas. For the case in which different transmission power levels areused for the active antennas, the normalized SAR distribution for eachactive antenna may be scaled by the respective transmission power levelbefore combining the normalized SAR distributions for the activeantennas. The normalized SAR distribution for simultaneous transmissionfrom multiple active antennas may be given by the following:

$\begin{matrix}{{SAR}_{norm\_ combined} = {\sum\limits_{i = 1}^{i = K}{\frac{{Tx}_{i}}{{Tx}_{SARi}} \cdot \frac{{SAR}_{i}}{{SAR}_{\lim}}}}} & (2)\end{matrix}$

where SAR_(lim) is a SAR limit, SAR_(norm_combined) is the combinednormalized SAR distribution for simultaneous transmission from theactive antennas, i is an index for the active antennas, SAR_(i) is theSAR distribution for the i^(th) active antenna, Tx_(i) is thetransmission power level for the i^(th) active antenna, Tx_(SARi) is thetransmission power level for the SAR distribution for the i^(th) activeantenna, and K is the number of the active antennas. Equation (2) may berewritten as follows:

$\begin{matrix}{{SAR}_{{norm\_ combine}d} = {\sum_{i = 1}^{i = K}{\frac{{Tx}_{i}}{{Tx}_{SARi}} \cdot {SAR}_{{norm\_}i}}}} & ( {3a} )\end{matrix}$

where SAR_(norm_i) is the normalized SAR distribution for the i^(th)active antenna. In the case of simultaneous transmissions using multipleactive antennas at the same transmitting frequency (e.g., multiple inmultiple out (MIMO)), the combined normalized SAR distribution isobtained by summing the square root of the individual normalized SARdistributions and computing the square of the sum, as given by thefollowing:

$\begin{matrix}{{SAR}_{{{norm\_ combine}d}{\_{MIMO}}} = {\lbrack {\sum_{i = 1}^{i = K}{\frac{{Tx}_{i}}{{Tx}_{SARi}} \cdot {SAR}_{{norm\_}i}}} \rbrack^{2}.}} & ( {3b} )\end{matrix}$

In another example, normalized SAR distributions for different frequencybands may be stored in the memory 115. In this example, a normalized SARdistribution for a transmit scenario in which two or more frequencybands are active may be generated by combining the normalized SARdistributions for the two or more active frequency bands. For the casewhere the transmission power levels are different for the activefrequency bands, the normalized SAR distribution for each of the activefrequency bands may be scaled by the respective transmission power levelbefore combining the normalized SAR distributions for the activefrequency bands. In this example, the combined SAR distribution may alsobe computed using equation (3a) in which i is an index for the activefrequency bands, SAR_(norm_i) is the normalized SAR distribution for thei^(th) active frequency band, Tx_(i) is the transmission power level forthe i^(th) active frequency band, and Tx_(SARi) is the transmissionpower level for the normalized SAR distribution for the i^(th) activefrequency band.

To assess RF exposure from transmissions using the second technology(e.g., 5G, IEEE 802.11ad, etc.), the wireless device 100 may includemultiple PD distributions for the second technology stored in the memory115. Each of the PD distributions may correspond to a respective one ofmultiple transmit scenarios supported by the wireless device 100 for thesecond technology. The transmit scenarios may correspond to variouscombinations of antennas 132-1 to 132-M, frequency bands, channelsand/or body positions, as discussed further below.

The PD distribution (also referred to as PD map) for each transmitscenario may be generated based on measurements (e.g., E-fieldmeasurements) performed in a test laboratory using a model of a humanbody. After the PD distributions are generated, the PD distributions arestored in the memory 115 to enable the processor 110 to assess RFexposure in real time, as discussed further below. Each PD distributionincludes a set of PD values, where each PD value may correspond to adifferent location (e.g., on the model of the human body).

The PD values in each PD distribution correspond to a particulartransmission power level (e.g., the transmission power level at whichthe PD values were measured in the test laboratory). Since PD scaleswith transmission power level, the processor 110 may scale a PDdistribution for any transmission power level by multiplying each PDvalue in the PD distribution by the following transmission power scaler:

$\begin{matrix}\frac{{Tx}_{c}}{{Tx}_{PD}} & (4)\end{matrix}$

where Tx_(c) is a current transmission power level for the respectivetransmit scenario, and T_(XPD) is the transmission power levelcorresponding to the PD values in the PD distribution (e.g., thetransmission power level at which the PD values were measured in thetest laboratory).

As discussed above, the wireless device 100 may support multipletransmit scenarios for the second technology. In certain aspects, thetransmit scenarios may be specified by a set of parameters. The set ofparameters may include one or more of the following: an antennaparameter indicating one or more antennas used for transmission (i.e.,active antennas), a frequency band parameter indicating one or morefrequency bands used for transmission (i.e., active frequency bands), achannel parameter indicating one or more channels used for transmission(i.e., active channels), a body position parameter indicating thelocation of the wireless device 100 relative to the user's body location(head, trunk, away from the body, etc.), and/or other parameters. Incases where the wireless device 100 supports a large number of transmitscenarios, it may be very time-consuming and expensive to performmeasurements for each transmit scenario in a test setting (e.g., testlaboratory). To reduce test time, measurements may be performed for asubset of the transmit scenarios to generate PD distributions for thesubset of transmit scenarios. In this example, the PD distribution foreach of the remaining transmit scenarios may be generated by combiningtwo or more of the PD distributions for the subset of transmitscenarios, as discussed further below.

For example, PD measurements may be performed for each one of theantennas 132-1 to 132-M to generate a PD distribution for each one ofthe antennas 132-1 to 132-M. In this example, a PD distribution for atransmit scenario in which two or more of the antennas 132-1 to 132-Mare active may be generated by combining the PD distributions for thetwo or more active antennas.

In another example, PD measurements may be performed for each one ofmultiple frequency bands to generate a PD distribution for each one ofthe multiple frequency bands. In this example, a PD distribution for atransmit scenario in which two or more frequency bands are active may begenerated by combining the PD distributions for the two or more activefrequency bands.

In certain aspects, a PD distribution may be normalized with respect toa PD limit by dividing each PD value in the PD distribution by the PDlimit. In this case, a normalized PD value exceeds the PD limit when thenormalized PD value is greater than one, and is below the PD limit whenthe normalized PD value is less than one. In these aspects, each of thePD distributions stored in the memory 115 may be normalized with respectto a PD limit.

In certain aspects, the normalized PD distribution for a transmitscenario may be generated by combining two or more normalized PDdistributions. For example, a normalized PD distribution for a transmitscenario in which two or more antennas are active may be generated bycombining the normalized PD distributions for the two or more activeantennas. For the case in which different transmission power levels areused for the active antennas, the normalized PD distribution for eachactive antenna may be scaled by the respective transmission power levelbefore combining the normalized PD distributions for the activeantennas. The normalized PD distribution for simultaneous transmissionfrom multiple active antennas may be given by the following:

$\begin{matrix}{{PD}_{{norm\_ combine}d} = {\sum_{i = 1}^{i = L}{\frac{{Tx}_{i}}{{Tx}_{PDi}} \cdot \frac{{PD}_{i}}{{PD}_{\lim}}}}} & (5)\end{matrix}$

where PD_(lim) is a PD limit, PD_(norm_combined) is the combinednormalized PD distribution for simultaneous transmission from the activeantennas, i is an index for the active antennas, PD_(i) is the PDdistribution for the i^(th) active antenna, Tx_(i) is the transmissionpower level for the i^(th) active antenna, T_(XPDi) is the transmissionpower level for the PD distribution for the i^(th) active antenna, and Lis the number of the active antennas. Equation (5) may be rewritten asfollows:

$\begin{matrix}{{PD}_{{norm\_ combine}d} = {\sum_{i = 1}^{i = L}{\frac{{Tx}_{i}}{{Tx}_{PDi}} \cdot {PD}_{norm\_ i}}}} & ( {6a} )\end{matrix}$

where PD_(norm_i) is the normalized PD distribution for the i^(th)active antenna. In the case of simultaneous transmissions using multipleactive antennas at the same transmitting frequency (e.g., MIMO), thecombined normalized PD distribution is obtained by summing the squareroot of the individual normalized PD distributions and computing thesquare of the sum, as given by the following:

$\begin{matrix}{{PD}_{{{norm\_ combine}d}{\_{MIMO}}} = {\lbrack {\sum_{i = 1}^{i = L}\sqrt{\frac{{Tx}_{i}}{{Tx}_{PDi}} \cdot {PD}_{{norm\_}i}}} \rbrack^{2}.}} & ( {6b} )\end{matrix}$

In another example, normalized PD distributions for different frequencybands may be stored in the memory 115. In this example, a normalized PDdistribution for a transmit scenario in which two or more frequencybands are active may be generated by combining the normalized PDdistributions for the two or more active frequency bands. For the casewhere the transmission power levels are different for the activefrequency bands, the normalized PD distribution for each of the activefrequency bands may be scaled by the respective transmission power levelbefore combining the normalized PD distributions for the activefrequency bands. In this example, the combined PD distribution may alsobe computed using equation (6a) in which i is an index for the activefrequency bands, PD_(norm_i) is the normalized PD distribution for thei^(th) active frequency band, Tx_(i) is the transmission power level forthe i^(th) active frequency band, and TX_(PDi) is the transmission powerlevel for the normalized PD distribution for the i^(th) active frequencyband.

As discussed above, the wireless device 100 may simultaneously transmitsignals using the first technology (e.g., 3G, 4G, IEEE 802.11ac, etc.)and the second technology (e.g., 5G, IEEE 802.11ad, etc.), in which RFexposure is measured using different metrics for the first technologyand the second technology (e.g., SAR for the first technology and PD forthe second technology). In this case, the processor 110 may determine afirst maximum allowable power level for the first technology and asecond maximum allowable power level for the second technology fortransmissions in a future time slot that comply with RF exposure limits.During the future time slot, the transmission power levels for the firstand second technologies are constrained (i.e., bounded) by thedetermined first and second maximum allowable power levels,respectively, to ensure compliance with RF exposure limits, as furtherbelow. In the present disclosure, the term “maximum allowable powerlevel” refers to a “maximum allowable power level” imposed by an RFexposure limit unless stated otherwise. It is to be appreciated that the“maximum allowable power level” is not necessarily equal to the absolutemaximum power level that complies with an RF exposure limit and may beless than the absolute maximum power level that complies with the RFexposure limit (e.g., to provide a safety margin). The “maximumallowable power level” may be used to set a power level limit on atransmission at a transmitter such that the power level of thetransmission is not allowed to exceed the “maximum allowable powerlevel” to ensure RF exposure compliance.

The processor 110 may determine the first and second maximum allowablepower levels as follows. The processor may determine a normalized SARdistribution for the first technology at a first transmission powerlevel, determine a normalized PD distribution for the second technologyat a second transmission power level, and combine the normalized SARdistribution and the normalized PD distribution to generate a combinednormalized RF exposure distribution (referred to simply as a combinednormalized distribution below). The value at each location in thecombined normalized distribution may be determined by combining thenormalized SAR value at the location with the normalized PD value at thelocation or another technique.

The processor 110 may then determine whether the first and secondtransmission power levels comply with RF exposure limits by comparingthe peak value in the combined normalized distribution with one. If thepeak value is equal to or less than one (i.e., satisfies thecondition≤1), then the processor 110 may determine that the first andsecond transmission power levels comply with RF exposure limits (e.g.,SAR limit and PD limit) and use the first and second transmission powerlevels as the first and second maximum allowable power levels,respectively, during the future time slot. If the peak value is greaterthan one, then the processor 110 may determine that the first and secondtransmission power levels do not comply with RF exposure limits. Toavoid non-compliance during the future time slot, the processor 110 mayreduce one or more of the first and second transmission power levels sothat the peak value in the combined normalized distribution is equal toor less than one. In this case, the processor 110 may use the first andsecond transmission power levels that comply with the RF exposure limitsas the first and second maximum allowable power levels, respectively,during the future time slot. The condition for RF exposure compliancefor simultaneous transmissions using the first and second technologiesmay be given by:

SAR_(norm)+PD_(norm)≤1   (7).

During the future time slot, the processor 110 limits (constrains) thetransmission power level of the first transmitter 120 by the firstmaximum allowable power level. For example, if a power control loop isused for the first technology, the power control loop is allowed to setthe transmission power level of the first transmitter 120 to a powerlevel equal to or below the first maximum allowable power level, but nota power level exceeding the first maximum allowable power level. Duringthe future time slot, the processor 110 also limits (constrains) thetransmission power level of the second transmitter 130 by the secondmaximum allowable power level. For example, if a power control loop isused for the second technology, the power control loop is allowed to setthe transmission power level of the second transmitter 130 to a powerlevel equal to or below the second maximum allowable power level, butnot a power level exceeding the second maximum allowable power level.

FIG. 2 shows a visually representation of the normalized SARdistribution 210 and the normalized PD distribution 220, in which thenormalized SAR distribution 210 and the normalized PD distribution 220are combined to generate a combined normalized distribution 230. FIG. 2also shows the condition that the peak value in the combined normalizeddistribution 230 be equal to or less than one for RF exposurecompliance. Although each of the distributions 210, 220 and 230 isdepicted as a two-dimensional distribution in FIG. 2, it is to beappreciated that the present disclosure is not limited to this example.

The normalized SAR distribution in equation (7) may be generated bycombining two or more normalized SAR distributions as discussed above(e.g., for a transmit scenario using multiple active antennas).Similarly, the normalized PD distribution in equation (7) may begenerated by combining two or more normalized PD distributions asdiscussed above (e.g., for a transmit scenario using multiple activeantennas). In this case, the condition for RF exposure compliance inequation (7) may be rewritten using equations (3a) and (6a) as follows:

$\begin{matrix}{{{\sum_{i = 1}^{i = K}{\frac{{Tx}_{i}}{{Tx}_{SARi}} \cdot {SAR}_{norm\_ i}}} + {\sum_{i = 1}^{i = L}{\frac{{Tx}_{i}}{{Tx}_{PDi}} \cdot {PD}_{{norm\_}i}}}} \leq 1.} & (8)\end{matrix}$

For the MIMO case, equations (3b) and (6b) may be combined instead. Asshown in equation (8), the combined normalized distribution may be afunction of transmission power levels for the first technology andtransmission power levels for the second technology. All the points inthe combined normalized distribution should meet the normalized limit ofone in equation (8). Additionally, when combining SAR and PDdistributions, the SAR and PD distributions should be aligned spatiallyor aligned with their peak locations so that the combined distributiongiven by equation (8) represents combined RF exposure for a givenposition of a human body.

For the case in which the wireless device 100 simultaneously transmitssignals using the first and second technologies, the processor 110 maydetermine one or more maximum allowable power levels for the firsttechnology and one or more maximum allowable power levels for the secondtechnology for transmissions in a future time slot as follows. Theprocessor 110 retrieves one or more normalized SAR distributions for thefirst technology from the memory 115 based on a transmit scenario forthe first technology in the future time slot and retrieves one or morenormalized PD distributions for the second technology from the memory115 based on a transmit scenario for the second technology in the futuretime slot. For example, if the transmit scenario for the firsttechnology uses multiple active antennas, then the processor 110 mayretrieve a normalized SAR distribution for each of the active antennas.Similarly, if the transmit scenario for the second technology usesmultiple active antennas, then the processor 110 may retrieve anormalized PD distribution for each of the active antennas.

The processor 110 may then determine maximum allowable power levels forthe first and second technologies that comply with the RF exposurelimits (e.g., SAR limit and PD limit) by performing the exemplary methodillustrated in FIG. 3.

At block 310, the processor 110 initializes the transmission powerlevels for the first and second technologies according to the transmitscenarios for the first and second technologies in the future time slot.If the transmit scenario for the first technology uses multiple activeantennas, then the transmission power levels may include a transmissionpower level for each of the active antennas for the first technology.Similarly, if the transmit scenario for the second technology usesmultiple active antennas, then the transmission power levels may includea transmission power level for each of the active antennas for thesecond technology.

The transmission power levels for the first and second technologies maybe initialized according to one or more power control loops, one or moredesired data rates, one or more desired beam directions or sectors, etc.In one example, the transmission power levels may be initialized to aset of default transmission power levels.

At block 320, the processor 110 determines a combined normalizeddistribution based on the transmission power levels in block 310, theretrieved normalized SAR distributions, and the retrieved normalized PDdistributions (e.g., according to equation (8) discussed above).

At block 330, the processor 110 compares the peak value in the combinednormalized distribution with one. If the peak value in the combinednormalized distribution is equal to or less than one (i.e., satisfiesthe condition 1), then the processor 110 determines that thetransmission power levels comply with RF exposure limits. In this case,the method 300 ends at block 350, and the processor 110 uses thetransmission power levels as the maximum allowable power levels for thefuture time slot.

If the peak value in the combined normalized distribution is greaterthan one, then the processor 110 adjusts the transmission power levelsat block 340. For example, the processor 110 may adjust the transmissionpower levels by reducing one or more of the transmission power levels.

The processor 110 then repeats block 320 and 330 using the adjustedtransmission power levels (i.e., determines the combined normalizeddistribution in block 320 using the adjusted transmission power levels).The processor 110 may repeat block 340, 320 and 330 until the peak valuein the combined normalized distribution is equal or less than one, atwhich point the transmission power levels comply with RF exposurelimits. The transmission power levels that comply with the RF exposurelimits are then used as the maximum allowable power levels for thefuture time slot. The maximum allowable power levels include one or moremaximum allowable power levels for the first technology and one or moremaximum allowable power levels for the second technology. For theexample in which multiple active antennas (e.g., two or more of antennas122-1 to 122-N) are used for the first technology, the maximum allowablepower levels include a maximum allowable power level for each of theactive antennas. For the example in which multiple active antennas(e.g., two or more of antennas 132-1 to 132-M) are used for the secondtechnology, the maximum allowable power levels include a maximumallowable power level for each of the active antennas.

After the processor 110 determines the maximum allowable power levels,the processor 110 constrains transmission power of the first transmitter120 during the future time slot by the one or more determined maximumallowable power levels for the first technology. For the example inwhich the first transmitter 120 transmits signals using multipleantennas (e.g., two or more of antennas 122-1 to 122-N) during thefuture time slot, the maximum allowable power levels include a maximumallowable power level for each of the active antennas. In this example,the processor 110 constrains the transmission power level for each ofthe active antennas by the respective maximum allowable power level. Theprocessor 110 also constrains transmission power of the secondtransmitter 130 during the future time slot by the one or moredetermined maximum allowable power levels for the second technology. Forthe example in which the second transmitter 130 transmits signals usingmultiple antennas (e.g., two or more of antennas 132-1 to 132-M) duringthe future time slot, the maximum allowable power levels include amaximum allowable power level for each of the active antennas. In thisexample, the processor 110 constrains the transmission power level foreach of the active antennas by the respective maximum allowable powerlevel.

It is to be appreciated that the present disclosure is not limited tothe exemplary method 300 illustrated in FIG. 3, and that other methodsmay be employed to determine maximum allowable power levels for thefirst and second technologies that comply with the RF exposure limits.For example, the processor 110 may determine maximum allowable powerlevels that result in the peak value in the combined normalizeddistribution being equal to or less than a value that is less than onefor a conservative approximate analysis to determine the maximumallowable power levels with fewer computations. Thus, a value of lessthan one may be used as the condition for assessing RF exposurecompliance.

In some cases, the wireless device 100 may transmit signals using thesecond technology (e.g., 5G, IEEE 802.11ad, etc.) when the firsttechnology is not active. In these cases, RF exposure from the firsttechnology does not need to be considered to assess RF exposurecompliance.

In these cases, the processor 110 may determine maximum allowable powerlevels for the second technology in a future time slot that comply witha PD limit as follows. First, the processor 110 may retrieve normalizedPD distributions for the second technology from the memory 115 based ona transmit scenario for the second technology in the future time slot.For example, if the transmit scenario for the second technology in thefuture time slot uses multiple active antennas, then the processor 110may retrieve a normalized PD distribution for each of the activeantennas. In this example, the active antennas may be selected, e.g.,based on a desired beam direction or sector for transmission by thewireless device 100 in the future time slot.

The processor 110 may then determine maximum allowable power levels forthe second technology that comply with the PD limit by performing theexemplary method illustrated in FIG. 4.

At block 410, the processor 110 initializes the transmission powerlevels for the second technology according to the transmit scenario forthe second technology. If the transmit scenario for the secondtechnology uses multiple active antennas, then the transmission powerlevels may include a transmission power level for each of the activeantennas. The transmission power levels may be initialized according toa power control loop, a desired data rate, a desired beam direction orsector, etc. In one example, the transmission power levels may beinitialized to a set of default transmission power levels.

At block 420, the processor 110 determines a combined normalized PDdistribution based on the transmission power levels in block 410, andthe retrieved normalized PD distributions (e.g., according to equation(6a) or (6b) discussed above).

At block 430, the processor 110 compares the peak value in the combinednormalized PD distribution with one. If the peak value in the combinednormalized PD distribution is equal to or less than one (i.e., satisfiesthe condition 1), then the processor 110 determines that thetransmission power levels comply with the PD limit. In this case, themethod 400 ends at block 450, and the processor 110 uses thetransmission power levels as the maximum allowable power levels for thesecond transmitter 130.

If the peak value in the combined normalized PD distribution is greaterthan one, then the processor 110 adjusts the transmission power levelsat block 440. For example, the processor 110 may adjust the transmissionpower levels by reducing one or more of the transmission power levelsinitialized in block 410.

The processor 110 then repeats block 420 and 430 using the adjustedtransmission power levels (i.e., determines the combined normalized PDdistribution in block 420 using the adjusted transmission power levels).The processor 110 may repeat blocks 440, 420 and 430 until the peakvalue in the combined normalized PD distribution is equal or less thanone, at which point the transmission power levels comply with the PDlimit. The processor 110 then uses the transmission power levels thatcomply with the PD limit as the maximum allowable power levels for thesecond transmitter 130. After the processor 110 determines the maximumallowable power levels complying with the PD limit, the processor 110constrains transmission power for the second transmitter 130 during thefuture time slot according to the determined maximum allowable powerlevels. For the example in which the second transmitter 130 transmitssignals using multiple active antennas (e.g., two or more of antennas132-1 to 132-M) during the future time slot, the maximum allowable powerlevels for the second technology include a maximum allowable power levelfor each of the active antennas. In this example, the processor 110constrains the transmission power level for each of the active antennasby the respective maximum allowable power level.

It is to be appreciated that the present disclosure is not limited tothe exemplary method 400 illustrated in FIG. 4, and that other methodsmay be employed to determine maximum allowable power levels that complywith the PD limit. For example, the processor 110 may determine maximumallowable power levels that that result in the peak value being equal toor less than a value that is less than one for a conservativeapproximate analysis to determine the maximum allowable power levelswith fewer computations.

In certain cases, an RF exposure regulation requires that atime-averaged RF exposure over a time window not exceed an RF exposurelimit. This allows the wireless device 100 to briefly exceed the RFexposure limit as long as the time-averaged RF exposure does not exceedthe limit.

In this regard, the processor 110 may determine RF exposure compliancefor the case in which the first technology is active and the secondtechnology is not active as follows. The processor 110 may compute atime-averaged normalized SAR distribution over a first time window(e.g., 6 minutes), and compare the peak value in the time-averagednormalized SAR distribution with one to assess RF exposure compliance.If the peak value is equal to or less than one (i.e., satisfies thecondition 1), then the processor 110 may determine RF exposurecompliance.

In this regard, FIG. 5 illustrates an example in which the processor 110computes a time-averaged normalized SAR distribution over a first timewindow 505 (e.g., 6 minutes). In this example, the first time window 505is divided into multiple time slots (i.e., time intervals). Forinstance, a 6-minute time window may be divided into 5-second timeslots. In the example shown in FIG. 5, there are p number of time slots515(1)-515(p), and p number of normalized SAR distributions510(1)-510(p). Although each of the distributions 510(1)-510(p) isdepicted as a two-dimensional distribution in FIG. 5, it is to beappreciated that the present disclosure is not limited to this example.

The processor 110 may determine a normalized SAR distribution for eachtime slot (e.g., according to equation (3a) or (3b)). The normalized SARdistribution for a time slot may be generated by combining two or moreSAR distributions. For example, if two or more antennas are activeduring the time slot, then the processor 110 may combine the normalizedSAR distributions for the two or more active antennas to generate thenormalized SAR distribution for the time slot. For the case in whichdifferent transmission power levels are used for the active antennas,the processor 110 may scale the normalized SAR distribution for eachactive antenna by the transmission power level for the antenna.

In certain aspects, the transmit scenario and/or transmission powerlevels for the first technology may vary over the first time window 505.In these aspects, the transmit scenario may be approximately constantover one time slot, but may vary from time slot to time slot within thefirst time window 505. The processor 110 may determine the normalizedSAR distribution for each time slot based on the transmit scenario andtime-averaged transmission power levels for the time slot (e.g.,according to equation (3a) or 3(b)).

The processor 110 may average the normalized SAR distributions510(1)-510(p) over the first time window 505 to generate a time-averagednormalized SAR distribution 520. For example, the processor 110 maycompute the time-averaged normalized SAR distribution 520 by combiningthe normalized SAR distributions 510(1)-510(p) for the time slots515(1)-515(p) and dividing the resulting combined normalized SARdistribution by the number of time slots as given by the following:

$\begin{matrix}{{\frac{1}{p}{\sum_{j = 1}^{j = p}{SAR}_{norm\_ j}}} \leq 1} & ( {9a} )\end{matrix}$

where SAR_(norm_i) represents the normalized SAR distribution for thej^(th) time slot 510(j). As discussed above, the normalized SARdistribution for a time slot may be a combination of multiple SARdistributions for the time slot (e.g., for the case of multiple activeantennas). The processor 110 may then compare the peak value in thetime-averaged normalized SAR distribution 520 with one to assess RFexposure compliance. If the peak value is equal to or less than one(i.e., satisfies the condition 1), then the processor 110 may determineRF exposure compliance.

In certain aspects, the processor 110 may determine maximum allowablepower levels for a future time slot to ensure time-average RF exposurecompliance. In this regard, the time slots 515(1)-515(p−1) in FIG. 5 maycorrespond to previous transmissions by the wireless device 100, and thetime slot 515(p) may correspond to the future time slot. In this regard,the time slot 515(p) is referred to as the future time slot below.Equation (9a) may be written as follows:

$\begin{matrix}{{\frac{1}{p}\lbrack {( {\sum_{j = 1}^{j = {p - 1}}{SAR}_{norm\_ j}} ) + {SAR}_{{norm\_}p}} \rbrack} \leq 1} & ( {9b} )\end{matrix}$

where SAR_(norm_p) is the SAR distribution for the future time slot515(p).

In this example, it is assumed that the transmission power levels forthe normalized SAR distributions 510(1)-510(p−1) are known by theprocessor 110 since they correspond to previous transmissions by thewireless device 100. For example, the processor 110 may record thetransmission power levels and transmit scenario for each of the timeslots 515(1)-515(p−1) in the memory 115, and use the recordedtransmission power levels and transmit scenarios for the time slots515(1)-515(p−1) to determine the normalized SAR distributions510(1)-510(p−1) for these time slots. For time slots 515(1)-515(p−1),the normalized SAR distribution for the j^(th) time slot 515(j) may bedetermined using equation (3a) or (3b) for all transmission scenariosand power levels that were active during the j^(th) time slot 515(j).

In this example, the transmission power levels for the normalized SARdistribution 510(p) corresponding to the future time slot 515(p) arevariables to be solved by the processor 110. To determine maximumallowable power levels for the future time slot 515(p), the processor110 may compute the time-averaged normalized SAR distribution 520 inwhich the transmission power levels for the future time slot 515(p) arevariables in the time-averaged normalized SAR distribution 520 (i.e.,the time-averaged normalized SAR distribution is a function of thetransmission power levels for the future time slot 515(p)). Theprocessor 110 may then determine transmission power levels for thefuture time slot 515(p) such that the peak value in the time-averagednormalized SAR distribution is equal to or less than one (i.e.,satisfies the condition 1 in equation (9b)). The processor 110 uses thetransmission power levels that satisfy the condition for RF exposurecompliance as the maximum allowable power levels for the future timeslot 515(p), and sets the transmission power limits for the future timeslot 515(p) according to the determined maximum allowable power levels.The processor 110 may determine the maximum allowable power levels forthe future time slot 515(p) during time slot 515(p−1) so that themaximum allowable power levels for the future time slot 515(p) are readyat the start of the future time slot 515(p) for the processor 110 toenforce the maximum allowable power levels.

The processor 110 may determine the maximum allowable power levels forthe future time slot 515(p) according to the exemplary method 600illustrated in FIG. 6. At block 610, the processor 110 initializes thetransmission power levels for the future time slot 515(p) according tothe transmit scenario for the future time slot 515(p). The transmissionpower levels may be initialized according to a power control loop, adesired data rate, a desired beam direction or sector, etc. In oneexample, the transmission power levels may be initialized to a set ofdefault transmission power levels.

At block 620, the processor 110 determines the time-averaged normalizedSAR distribution based on the transmit scenario and transmission powerlevels at block 610 for the future time slot 515(p). Note that thetransmission power levels for the previous time slots 515(1)-515(p−1)are known, as discussed above.

At block 630, the processor 110 compares the peak value in thetime-averaged normalized

SAR distribution with one to assess RF exposure compliance. If the peakvalue is equal to or less than one, then the method 600 ends at block650. In this case, the processor 110 uses the transmission power levelsinitialized at block 610 as the maximum allowable power levels for thefuture time slot 515(p).

If the peak value is greater than one, then the processor 110 adjuststhe transmission power levels for the future time slot at block 640. Theprocessor 110 may adjust the transmission power levels for the futuretime slot by reducing one or more of the transmission power levels forthe future time slot. The processor 110 then repeats blocks 620 and 630using the adjusted transmission power levels. The processor 110 mayrepeat blocks 640, 620 and 630 until the peak value in the time-averagednormalized SAR distribution is equal or less than one, at which pointthe transmission power levels comply with the SAR limit and theprocessor 110 uses the transmission power levels that comply with theSAR limit as the maximum allowable power levels for the future time slot515 (p).

For the example in which the first transmitter 120 transmits signalsusing multiple active antennas (e.g., two or more of antennas 122-1 to122-N) during the future time slot 515(p), the maximum allowable powerlevels may include a maximum allowable power level for each of theactive antennas. In this example, the processor 110 limits (constrains)the transmission power level for each of the active antennas by therespective maximum allowable power level.

It is to be appreciated that the present disclosure is not limited tothe exemplary method 600 illustrated in FIG. 6, and that other methodsmay be employed to determine transmission power levels for the futuretime slot 515(p) such that the time-averaged normalized SAR distributioncomplies with the SAR limit. For example, the processor 110 maydetermine maximum allowable power levels that result in the peak valueof the time-averaged normalized SAR distribution being equal to or lessthan a value that is less than one for a conservative approximateanalysis to determine the maximum allowable power levels with fewercomputations.

In certain cases, a regulator may require that a time-averaged PDdistribution for the second technology not exceed a PD limit for thesecond technology. This allows the wireless device 100 to briefly exceedthe PD limit as long as the time-averaged PD distribution does notexceed the PD limit.

In this regard, the processor 110 may determine RF exposure compliancefor the case in which the second technology is active and the firsttechnology is not active as follows. The processor 110 may compute atime-averaged normalized PD distribution over a second time window(e.g., 2 minutes), and compare the peak value in the time-averagednormalized PD distribution with one to assess RF exposure compliance. Ifthe peak value is equal to or less than one (i.e., satisfies thecondition 1), then the processor 110 may determine RF exposurecompliance.

In this regard, FIG. 7 illustrates an example in which the processor 110computes a time-averaged normalized PD distribution over a second timewindow 705 (e.g., 2 minutes). In this example, the second time window705 is divided into multiple time slots (i.e., time intervals). Forinstance, a 2-minute time window may be divided into 5-second timeslots. In the example shown in FIG. 7, there are q number of time slots715(1)-715(q), and q number of normalized PD distributions710(1)-710(q). Although each of the distributions 710(1)-710(q) isdepicted as a two-dimensional distribution in FIG. 7, it is to beappreciated that the present disclosure is not limited to this example.

The processor 110 may determine a normalized PD distribution for eachtime slot (e.g., according to equation (6a) or (6b)). The normalized PDdistribution for a time slot may be generated by combining two or morePD distributions. For example, if two or more antennas are active duringthe time slot, then the processor 110 may combine the normalized PDdistributions for the two or more active antennas to generate thenormalized PD distribution for the time slot. For the case in whichdifferent transmission power levels are used for the active antennas,the processor 110 may scale the normalized PD distribution for eachactive antenna by the respective transmission power level.

In certain aspects, the transmit scenario and/or transmission powerlevels for the second technology may vary over the second time window705. In these aspects, the transmit scenario may be approximatelyconstant over one time slot, but may vary from time slot to time slotwithin the second time window 705. The processor 110 may determine thenormalized PD distribution for each time slot based on the transmitscenario and time-averaged transmission power levels during the timeslot (e.g., according to equation (6a) or (6b)).

The processor 110 may average the normalized PD distributions710(1)-710(q) over the second time window 705 to generate atime-averaged normalized PD distribution 720. For example, the processor110 may compute the time-averaged normalized PD distribution 720 bycombining the normalized PD distributions 710(1)-710(q) for the timeslots 715(1)-715(q) and dividing the resulting combined normalized PDdistribution by the number of time slots as given by the following:

$\begin{matrix}{{\frac{1}{q}{\sum_{j = 1}^{j = q}{PD}_{{norm\_}i}}} \leq 1} & ( {10a} )\end{matrix}$

where PD_(nom_j) represents the normalized PD distribution for thej^(th) time slot 710(j). As discussed above, the normalized PDdistribution for a time slot may be a combination of multiple normalizedPD distributions for the time slot (e.g., for the case of multipleactive antennas). The processor 110 may then compare the peak value inthe time-averaged normalized PD distribution 720 with one to assess RFexposure compliance. If the peak value is equal to or less than one(i.e., satisfies the condition 1), then the processor 110 may determineRF exposure compliance.

In certain aspects, the processor 110 may determine maximum allowablepower levels for a future time slot to ensure time-averaged RF exposurecompliance. In this regard, the time slots 715(1)-715(q−1) in FIG. 7 maycorrespond to previous transmissions by the wireless device 100, and thetime slot 715(q) may correspond to a future transmission. In thisregard, the time slot 715(q) is referred to as the future time slotbelow. Equation (10a) may be rewritten as follows:

$\begin{matrix}{{\frac{1}{q}\lbrack {( {\sum_{j = 1}^{j = {q - 1}}{PD}_{norm\_ j}} ) + {PD}_{norm\_ q}} \rbrack} \leq 1} & ( {10b} )\end{matrix}$

where PD_(norm_q) is the normalized PD distribution for the future timeslot 715(q).

In this example, it is assumed that the transmission power levels forthe normalized PD distributions 710(1)-710(q−1) are known by theprocessor 110 since they correspond to previous transmissions by thewireless device 100. For example, the processor 110 may record thetransmission power levels and transmit scenario for each of the timeslots 715(1)-715(q−1) in the memory 115, and use the recordedtransmission power levels and transmit scenarios for the time slots715(1)-715(q−1) to determine the normalized PD distributions710(1)-710(q−1) for these time slots. For time slots 715(1)-715(q−1),the normalized PD distribution 710(j) for the j^(th) time slot 715(j)may be determined determined using equation (6a) or (6b) for alltransmission scenarios and power levels that were active during thej^(th) time slot 715(j).

In this example, the transmission power levels for the normalized PDdistribution 710(q) corresponding to the future time slot 715(q) arevariables to be solved by the processor 110. To determine the maximumallowable power levels for the future time slot 715(q), the processor110 may compute the time-averaged normalized PD distribution 720 inwhich the transmission power levels for the future time slot 715(q) arevariables in the time-averaged normalized PD distribution 720 (i.e., thetime-averaged normalized PD distribution 720 is a function of thetransmission power levels for the future time slot 715(q)). Theprocessor 110 may then determine transmission power levels for thefuture time slot 715(q) such that the peak value in the time-averagednormalized PD distribution is equal to or less than one (i.e., satisfiesthe condition 1 in equation (10 b)). The determined transmission powerlevels that comply with RF exposure levels are used as the maximumallowable power levels for the future time slot 715(q). In this regard,the processor 110 sets the transmission power limits for the future timeslot 715(q) according to the determined maximum allowable power levels.The processor 110 may determine the maximum allowable power levels forthe future time slot 715(q) during time slot 715(q−1) so that themaximum allowable power levels for the future time slot 715(q) are readyat the start of the future time slot 715(q) for the processor 110 toenforce the maximum allowable power levels.

The processor 110 may determine the maximum allowable power levels forthe future time slot 715(q) according to the exemplary method 800illustrated in FIG. 8. At block 810, the processor 110 initializes thetransmission power levels for the future time slot 715(q) according tothe transmit scenario for the future time slot 715(q). For example, thetransmission power levels may be initialized according to a powercontrol loop, a desired data rate, a desired beam direction or sector,etc. In one example, the transmission power levels may be initialized toa set of default transmission power levels.

At block 820, the processor 110 determines the time-averaged normalizedPD distribution based on the transmit scenario and transmission powerlevels at block 810 for the future time slot 715(q). Note that thetransmission power levels for the previous time slots 715(1)-715(q−1)are known, as discussed above.

At block 830, the processor 110 compares the peak value in thetime-averaged normalized

PD distribution with one to assess RF exposure compliance. If the peakvalue is equal to or less than one, then the method 800 ends at block850. In this case, the processor 110 uses the transmission power levelsinitialized at block 810 as the maximum allowable power levels for thefuture time slot 715(q).

If the peak value is greater than one, then the processor 110 adjuststhe transmission power levels 810 for the future time slot at block 840.The processor 110 may adjust the transmission power levels for thefuture time slot 715(q) by reducing one or more of the transmissionpower levels for the future time slot 715(q). The processor 110 thenrepeats blocks 820 and 830 using the adjusted transmission power levels.The processor 110 may repeat blocks 840, 820 and 830 until the peakvalue in the time-averaged PD distribution is equal or less than one, atwhich point the transmission power levels comply with the PD limit andthe processor 110 uses the transmission power levels as the maximumallowable power levels for the future time slot 715(q).

For the example in which the second transmitter 130 transmits signalsusing multiple active antennas (e.g., two or more of antennas 132-1 to132-M) during the future time slot 715(q), the maximum allowable powerlevels may include a maximum allowable power level for each of theactive antennas. In this example, the processor 110 limits (constrains)the transmission power level for each of the active antennas by therespective maximum allowable power level.

It is to be appreciated that the present disclosure is not limited tothe exemplary method 800 illustrated in FIG. 8, and that other methodsmay be employed to determine transmission power levels for the futuretime slot 715(q) such that the time-averaged normalized PD distributioncomplies with the PD limit. For example, the processor 110 may determinemaximum allowable power levels for the future time slot 715(q) thatresult in the peak value in the time-averaged normalized PD distributionbeing approximately equal to or less than a value that is less than one.

The processor 110 may also determine time-averaged RF exposurecompliance for the case where both the first technology and the secondtechnology are active (i.e., the wireless device simultaneouslytransmits signals using the first and second technologies). To do this,the processor 110 may combine the time-averaged normalized SARdistribution 520 and the time-averaged normalized PD distribution 720 togenerate a combined time-averaged normalized distribution 920, asillustrated in FIG. 9. The processor 110 may then compare the peak valuein the combined time-averaged normalized distribution 920 with one toassess time-averaged RF exposure compliance. If the peak value is equalto or less than one (i.e., satisfies the condition 1), then theprocessor 110 may determine that the wireless device 100 is compliant.The condition for compliance may be given by combining equations (9b)and (10b) as follows:

$\begin{matrix}{{{\frac{1}{p}\lbrack {( {\sum_{j = 1}^{j = {p - 1}}{SAR}_{norm\_ j}} ) + {SAR}_{{norm\_}p}} \rbrack} + {\frac{1}{q}\lbrack {( {\sum_{j = 1}^{j = {q - 1}}{PD}_{norm\_ j}} ) + {PD}_{norm\_ q}} \rbrack}} \leq 1.} & (11)\end{matrix}$

The first time window 505 for the time-averaged normalized SARdistribution and the second time window 705 for the time-averagednormalized PD distribution may be different in length. In this regard,FIG. 9 shows an example in which the first time window 505 is longerthan the second time window 705. For example, the first time window 505may be approximately 6 minutes in length and the second time window 705may be approximately 2 minutes in length. The lengths of the first andsecond time windows may be specified by respective RF exposureregulations (e.g., established by the FCC or other regulatory body).Note that the lengths of the time windows 505 and 705 are not drawn toscale in FIG. 9.

In certain aspects, the processor 110 may determine maximum allowablepower levels for the future time slots 515(p) and 715(q) of the firstand second technologies to ensure time-averaged RF exposure compliance.In these aspects, the future time slots 515(p) and 715(q) may beapproximately aligned in time as shown in the example in FIG. 9. Todetermine the maximum allowable power levels for the future time slots515(p) and 715(q), the processor 110 may compute the combinedtime-averaged normalized distribution 920 in which the transmissionpower levels for the future time slots 515(p) and 715(q) are variablesin the combined time-averaged normalized distribution 920 (i.e., thecombined time-averaged normalized distribution 920 is a function of thetransmission power levels for the future time slots 515(p) and 715(q)).The processor 110 may then determine the maximum allowable power levelsfor the future time slots 515(p) and 715(q) such that the peak value inthe combined time-averaged normalized distribution 920 is equal to orless than one (i.e., satisfies the condition 1 in equation (11)). Theprocessor 110 may then set the transmission power limits for the futuretime slots 515(p) and 715(q) according to the determined maximumallowable power levels.

The processor 110 may determine the maximum allowable power levels forthe future time slots 515(p) and 715(q) according to the exemplarymethod 1000 illustrated in FIG. 10. The maximum allowable power levelsmay include first maximum allowable power levels for the firsttechnology and second maximum allowable power levels for the secondtechnology.

At block 1010, the processor 110 initializes the transmission powerlevels for the future time slots 515(p) and 715(q) according to thetransmit scenarios for the future time slots 515(p) and 715(q). If thetransmit scenario for the future time slot 515(p) for the firsttechnology uses multiple active antennas, then the transmission powerlevels may include a transmission power level for each of the activeantennas. Similarly, if the transmit scenario for the future time slot715(q) for the second technology uses multiple active antennas, then thetransmission power levels may include a transmission power level foreach of the active antennas.

The transmission power levels may be initialized according to one ormore power control loops, one or more desired data rates, one or moredesired beam directions or sectors, etc. In one example, thetransmission power levels may be initialized to a set of defaulttransmission power levels.

At block 1020, the processor 110 determines the combined time-averagednormalized distribution 920 based on the transmission power levels forthe future time slots 515(p) and 715(q). Note that the transmissionpower levels in the previous time slots 515(1)-515(p-1) for the firsttechnology and the transmission power levels in the previous time slots715(1)-715(q−1) for the second technology are known, as discussed above.

At block 1030, the processor 110 compares the peak value in the combinedtime-averaged normalized distribution with one to assess RF exposurecompliance. If the peak value is equal to or less than one, then themethod 1000 ends at block 1050. In this case, the processor 110 uses thetransmission power levels initialized at block 1010 as the maximumallowable power levels for the future time slots 515(p) and 715(q).

If the peak value is greater than one, then the processor 110 adjuststhe transmission power levels for the future time slots at block 1040.The processor 110 may adjust the transmission power levels for thefuture time slots by reducing one or more of the transmission powerlevels for the future time slots. The processor 110 then repeats blocks1020 and 1030 using the adjusted transmission power levels. Theprocessor 110 may repeat blocks 1040, 1020 and 1030 until the peak valuein the combined time-averaged normalized distribution is equal or lessthan one, at which point the transmission power levels for the futuretime slots are compliant and the processor 110 uses the transmissionpower levels as the maximum allowable power levels. The determinedmaximum allowable power levels include the first maximum allowable powerlevels for the first technology and the second maximum allowable powerlevels for the second technology. In this regard, the processor 110 setsthe transmission power limits for the first transmitter 120 according tothe first maximum allowable power levels and sets the transmission powerlimits for the second transmitter 130 according to the second maximumallowable power levels.

It is to be appreciated that the present disclosure is not limited tothe exemplary method 1000 illustrated in FIG. 10, and that other methodsmay be employed to determine maximum allowable power levels for thefuture time slots 515(p) and 715(q) such that the combined time-averagednormalized distribution 920 complies with the RF exposure limits.

In certain aspects, the time-averaging window for PD is dependent ontransmitting frequency (e.g., ˜2 minutes in 28 GHz band and ˜1 minute in60 GHz). In these aspects, when the second transmitter 130 transmitssignals at multiple frequency bands, the time-average PD distributionmay be computed using a different time window for each frequency band.For example, if the second transmitter 130 transmits signals at a firstfrequency band (e.g., 28 GHz) and a second frequency band (e.g., 60GHz), the time-average PD distribution may be given by:

$\begin{matrix}{{\frac{1}{q}{\sum_{j = 1}^{j = q}{PD}_{norm\_ j}}} + {\frac{1}{r}{\sum_{j = 1}^{j = r}{PD}_{norm\_ j}}}} & (12)\end{matrix}$

where q is the number of time slots for the first frequency band (e.g.,28 GHz band), and r is the number of time slots for the second frequencyband (e.g., 60 GHz band). Since different time windows are used for thefirst and second frequency bands, the number of time slots for thefrequency band is different from the number of time slots for the secondfrequency band (i.e., q and r are different).

FIG. 11 shows an example in which two time-averaging windows are usedfor PD. In this example, the second time window 705 discussed above isused for the first frequency band (e.g., 28 GHz band) and a third timewindow 1105 is used for the second frequency band (e.g., 60 GHz band),in which the third time window 1105 is shorter than the second timewindow 705. For example, the second time window 705 may have a length ofapproximately two minutes and the third time window 1105 may have alength of approximately one minute.

As shown in FIG. 11, the third time window 1105 is divided into r numberof time slots 1115(1) to 1115(r). There are r number of normalized PDdistributions 1110(1) to 1110(r) for the second frequency band, whereeach normalized PD distribution corresponds to a respective one of thetime slots 1115(1) to 1115(r). In this example, time slots 1115(1) to1115(r-1) correspond to previous time slots and time slot 1115(r)corresponds to a future time slot approximately aligned with future timeslots 515(p) and 715(q).

In this example, the normalized PD distribution for each of the previoustime slots 715(1) to 715(q−1) in the second time window 705 may bedetermined based on the transmit scenario and transmission power levelsfor the first frequency band during the time slot. The normalized PDdistribution for the future time slot 715(q) is a function oftransmission power levels for the first frequency band in the futuretime slot 715(q). Similarly, the normalized PD distribution for each ofthe previous time slots 1115(1) to 1115(r-1) in the third time window1105 may be determined based on the transmit scenario and transmissionpower levels for the second frequency band during the time slot. Thenormalized PD distribution for the future time slot 1115(r) is afunction of transmission power levels for the second frequency band inthe future time slot 1115(r).

The time-average normalized PD distribution 720 may be computedaccording to equation (12) above, in which the time-average normalizedPD distribution is a function of the transmission power levels for thefirst frequency band in the future time slot 715(q) and the transmissionpower levels for the second frequency band in the future time slot1115(r).

In this example, the time-average normalized PD distribution 720 is acombination of the time-average normalized PD distribution for the firstfrequency band corresponding to the second time window 705 and thetime-average normalized PD distribution for the second frequency bandcorresponding to the third time window 1105. In this regard, thetime-average PD distribution 720 may be considered a combinedtime-averaged PD distribution.

For the example in which the wireless device 100 also transmits signalsusing the first technology, the time-averaged normalized PD distributionmay be combined with the time-averaged normalized SAR distribution toobtain the combined time-averaged normalized distribution discussedabove. In this example, the combined time-averaged normalizeddistribution is a function of the transmit scenario and transmissionpower levels for the first technology in the future time slot 515(p),the transmit scenario and transmission power levels for the firstfrequency band in the future time slot 715(q), and the transmit scenarioand transmission power levels for the second frequency band in thefuture time slot 1115(r). The maximum allowable power levels may bedetermined by determining transmission power levels that result in thepeak value of the combined time-averaged normalized distribution beingequal to or less than one (e.g., according to the method 1000illustrated in FIG. 10). In this example, the maximum allowable powerlevels include maximum allowable power levels for the first technology,maximum allowable power levels for the first frequency band, and maximumallowable power levels for the second frequency band. During the futuretime slots 515(p), 715(q) and 1115(r), the processor 110 sets thetransmission power limits for the first technology according to themaximum allowable power levels for the first technology, sets thetransmission power limits for the first frequency band according to themaximum allowable power levels for the first frequency band, and setsthe transmission power limits for the second frequency band according tothe maximum allowable power levels for the second frequency band.

Although two time-averaging widows 705 and 1105 are used for PD in theabove example, it is to be appreciated that more than two time-averagingwidows may be used depending on the number of different frequency bandsabove 10 GHz that are active. In general, the number of time-averagingwindows used for PD may be equal to the number of active frequency bandsabove 10 GHz, where each time-averaging window corresponds to arespective one of the active frequency bands.

In certain aspects, the wireless device 110 may simultaneously transmitsignals at the first and second frequency bands (e.g., 28 GHz and 60GHz) while the first technology is not active. In this case, theprocessor 110 may determine maximum allowable power levels for the firstand second frequency bands as follows. The processor may determine thetime-average normalized PD distribution according to equation (12) inwhich the time-average normalized PD distribution is a function of thetransmission power levels for the first frequency band in future timeslot 715(q) and the transmission power levels for the second frequencyband in future time slot 1115(r). An example of this is illustrated inFIG. 12, in which the condition for RF exposure compliance is that thetime-averaged normalized PD distribution 720 be equal to or less thanone. Note that the time-average normalized PD distribution is notcombined with the time-averaged normalized SAR distribution 520 in thiscase since the first technology is not active in this case.

The processor 110 may then determine transmission power levels for thefirst frequency band and second frequency band that result in the peakvalue in the time-average normalized PD distribution being equal to orless than one, and use the determine transmission power levels as themaximum allowable power levels. In this example, the maximum allowablepower levels include maximum allowable power levels for the firstfrequency band, and maximum allowable power levels for the secondfrequency band. During the future time slots 715(q) and 1115(r), theprocessor 110 sets the transmission power limits for the first frequencyband according to the maximum allowable power levels for the firstfrequency band and sets the transmission power limits for the secondfrequency band according to the maximum allowable power levels for thesecond frequency band. It is to be appreciated that the above techniquemay be expanded to three or more frequency bands to determine maximumallowable power levels for three or more frequency bands.

Some RF exposure regulations may not require time averaging of PD or maynot currently specify time-averaging for PD (which could change). Inthese cases, the time-averaged SAR distribution may be combined with anormalized PD distribution to assess RF exposure compliance. An exampleof this is illustrated in FIG. 13 in which the time-averaged normalizedSAR distribution 520 is combined with the normalized PD distribution forthe future time slot 715(q) to obtain the combined normalizeddistribution 920.

In this example, the processor 110 may determine maximum allowable powerlevels for the first and second technologies as follows. The processor110 combines the time-averaged normalized SAR distribution 520 with thenormalized PD distribution 710 for the future time slot 715(q) to obtainthe combined normalized distribution 920 in which the combinednormalized distribution 920 is a function of the transmission powerlevels for the first technology in future time slot 515(p) and thetransmission power levels for the second technology in future time slot715(q).

The processor 110 may then determine transmission power levels for thefirst and second technologies that result in the peak value in thenormalized distribution 920 being equal to or less than one, and use thedetermined transmission power levels as the maximum allowable powerlevels. In this example, the maximum allowable power levels includemaximum allowable power levels for the first technology, and maximumallowable power levels for the second technology. During the future timeslots 515(p) and 715(a), the processor 110 sets the transmission powerlimits for the first transmitter 120 according to the maximum allowablepower levels for the first technology and sets the transmission powerlimits for the second transmitter 130 band according to the maximumallowable power levels for the second technology.

It is to be appreciated that the time slots 515(1)-515(p) discussedabove may be equal in length or that two or more of the time slots515(1)-515(p) may have different lengths. The future time slot 515(p)may also be referred to as a time interval and may have a length that isequal to or less than a tenth the length of the first time window 505.In one example, the future time slot 515(p) has a length ofapproximately 5 seconds and the first time window 505 has a length ofapproximately 6 minutes.

It is to be appreciated that the time slots 715(1)-715(q) discussedabove may be equal in length or that two or more of the time slots715(1)-715(q) may have different lengths. The future time slot 715(q)may also be referred to as a time interval and may have a length that isequal to or less than a fifth the length of the second time window 705.In one example, the future time slot 715(q) has a length ofapproximately 5 seconds and the second time window 705 has a length ofapproximately 2 minutes.

It is to be appreciated that the time slots 1115(1)-1115(r) discussedabove may be equal in length or that two or more of the time slots1115(1)-1115(r) may have different lengths. The future time slot 1115(r)may also be referred to as a time interval and may have a length that isequal to or less than a fifth the length of the third time window 1105.In one example, the future time slot 1115(r) has a length ofapproximately 5 seconds and the third time window 1105 has a length ofapproximately one minute.

The first time window 505 may have a length that is at least 50 percentlonger than the length of the second time window 705. In one example,the first time window 505 has a length of approximately 6 minutes, andthe second time window 705 has a length of approximately 2 minutes, inwhich the lengths of the first and second time windows may be set byregulatory bodies. It is to be appreciated that the lengths of the firstand second time windows 505 and 705 set by the regulator bodies maychange with time, and may vary between different regulatory bodies. Asdiscussed above, regulatory bodies may define time windows that aredependent on transmitting frequency, for example, time-window length of2 minutes for 28 GHz band and time-window length of one minute for 60GHz bands. In that case, it is to be also appreciated that there couldbe one time window for SAR and two or more time windows for PD with eachtime window for PD corresponding to a given transmission frequency band.

As used herein, the term “previous time slots” refers to time slotsprior to the respective future time slot. For example, time slots515(1)-515(p−1) in FIG. 5 are previous time slots, which are prior tofuture time slot 515(p).

As used herein, the term “future time slot” refers to a time slot (i.e.,time interval or time duration) in the future with respect to the timethat the respective maximum allowable power levels are determined.Determining the maximum allowable power levels for the future time slotbefore the future time slot helps ensure RF exposure compliance duringthe future time slot. Since the future time slots 515(p), 715(q) and1115(r) discussed above are approximately aligned in time, they maycollectively be considered one future time slot.

It is to be appreciated that the time windows discussed above (e.g.,time windows 505, 705 and 1105) may be moving time-averaging windows. Inthis case, each time window is shifted by one time slot each time themaximum allowable power levels for a new future time slot aredetermined. For example, in the above discussion of time window 505,time slot 515(p) is given as the future time slot. To determine themaximum allowable power levels for the next future time slot 515(p+1),the processor 110 shifts the time window 505 by one time slot to covertime slots 515(2) to 515(p+1). Note that the first time slot 515(1) inthe previous determination of the maximum allowable power levels isdropped from the time window 505, and the future time slot 515(p) in theprevious determination of the maximum allowable power levels becomes thelast one of the previous time slots in the time window 505.

It is to be appreciated that the first communication technologydiscussed above may include multiple communication technologies in whichSAR is used to assess RF exposure compliance. For example, the firsttechnology may include WWAN, WLAN, Bluetooth, etc. In this regard, it isto be appreciated that the first transmitter 120 may include multipletransmitters. Also, it is to be appreciated that SAR may havecontributions from multiple sub-6GHz communication technologies (e.g.,simultaneous transmissions of WWAN, WLAN and Bluetooth).

It is to be appreciated that the second communication technologydiscussed above may include multiple communication technologies in whichPD is used to assess RF exposure compliance. For example, the secondtechnology may include mmWave/5G and mmWave/802.11ad. In this regard, itis to be appreciated that the second transmitter 130 may includemultiple transmitters. Also, it is to be appreciated that PD may havecontributions from multiple communication technologies (e.g.,simultaneous transmissions of mmWave/5G and mmWave/802.11ad).

In some of the examples given above, a normalized distribution iscompared with one to assess RF exposure compliance. However, it is to beappreciated that the present disclosure is not limited to theseexamples. For example, a distribution (e.g., SAR distribution, PDdistribution, combined RF exposure distribution, etc.) may be normalizedrelative to any value such that a limit value other than one may be usedto define the condition for RF exposure compliance. In this example, thecondition for RF compliance is that the normalized distribution be equalto or less than the limit value. Also, as discussed above, the limitvalue may be set to a value less than one.

As discussed above, the processor 110 may determine a maximum allowablepower level for a transmitter (e.g., the first transmitter 120 or thesecond transmitter 130) for a future time slot (e.g., according to anyof the methods described herein) and set a transmission power limit forthe transmitter based on the determined maximum allowable power level.In certain aspects, setting the transmission power limit based on thedetermined maximum allowable power level prevents a power level of thetransmitter from exceeding the maximum allowable power level at any timeduring the future time slot. In certain aspects, setting thetransmission power limit based on the determined maximum allowable powerlevel prevents a time-average of a power level of the transmitter overthe future time slot from exceeding the maximum allowable power level.This allows the power level to temporarily exceed the maximum allowablepower level within the future time slot as long as the time-average ofthe power level over the future time slot does not exceed the maximumallowable power level. In these aspects, the power level may exceed themaximum allowable power level for a time interval shorter than thefuture time slot. In these aspects, the maximum allowable power level isa maximum allowable time-average power level over the future time slot.

FIG. 14 illustrates an exemplary method 1400 implemented in a wirelessdevice (e.g., wireless device 100) according to certain aspects of thepresent disclosure.

At block 1410, a specific absorption rate (SAR) distribution for a firstwireless communication technology is determined. For example, the SARdistribution may comprise a time-averaged SAR distribution correspondingto a time-averaging window (e.g., time window 505). The first wirelesscommunication technology may include one or more of the following: WWAN,WLAN, 3G, 4G, Bluetooth, etc.

At block 1420, a power density (PD) distribution for a second wirelesscommunication technology is determined. For example, the PD distributionmay comprise a time-averaged PD distribution corresponding to one ormore time-averaging windows (e.g., time windows 705 and/or 1105). Thesecond wireless communication technology may include one or more of thefollowing: 5G, IEEE 802.11ad, etc.

At block 1430, the SAR distribution and the PD distribution are combinedto generate a combined RF exposure distribution.

At block 1440, at least one first maximum allowable power level and atleast one second maximum allowable power level are determined for afuture time slot based on the combined RF exposure distribution. Forexample, the combined RF exposure distribution may be a function oftransmission power levels in the future time slot. In this example, theat least one first maximum allowable power level and the at least onesecond maximum allowable power levels may be determined by determiningpower levels for the transmission power levels that result in a peakvalue of the combined RF exposure distribution being equal to or lessthan a limit value (e.g., limit value of one) that ensures RF exposurecompliance.

At block 1450, at least one transmission power limit for a firsttransmitter in the future time slot is set based on the at least onefirst maximum allowable power level. Setting the at least transmissionpower limit for the first transmitter (e.g., first transmitter 120)based on the at least one first maximum allowable power level may limittransmission power levels of the first transmitter in the future timeslot to the at least one first maximum allowable power level or limit atime-averaged transmission power level of the first transmitter over thefuture time slot to the at least one first maximum allowable powerlevel.

At block 1460, at least one transmission power limit for a secondtransmitter in the future time slot is set based on the at least onesecond maximum allowable power level. Setting the at least transmissionpower limit for the second transmitter (e.g., second transmitter 130)based on the at least one second maximum allowable power level may limittransmission power levels of the second transmitter in the future timeslot to the at least one second maximum allowable power level or limit atime-averaged transmission power level of the second transmitter overthe future time slot to the at least one second maximum allowable powerlevel.

It is to be appreciated that the method 1400 is not limited to theexemplary order shown in FIG. 14. For example, block 1420 may beperformed before block 1410 or both blocks 1410 and 1420 may beperformed contemporaneously. Also, block 1460 may be performed beforeblock 1450 or both blocks 1450 and 1460 may be performedcontemporaneously.

In the exemplary time-averaging approach discussed above with referenceto FIG. 9, the processor 110 determines a time-averaged SAR distribution505 for the first technology over a first time window 505 and determinesa time-averaged PD distribution 705 for the second technology over asecond time window 705, in which the first time window 505 and thesecond time window 705 have different time durations (i.e., differentlengths). The processor 110 then combines the time-averaged SARdistribution 505 and the time-averaged PD distribution 705 to obtain acombined time-averaged distribution 920, and determines a maximumallowable power level for the first technology and a maximum allowablepower level for the second technology for a future time slot based onthe combined time-averaged distribution 920. The processor 110 may makethis determination every Δt seconds, where Δt seconds (e.g., 5 seconds)is the time duration of one time slot. In this case, the processor 110updates the maximum allowable power level for the first technology andthe maximum allowable power level for the second technology every Δtseconds (e.g., 5 seconds), and the first time window 505 and the secondtime window 705 are moving time-averaging windows, as discussed above.In the example shown in FIG. 9, the same time-slot duration (e.g., 5seconds) is used for the first technology and the second technology.

The maximum peak-to-average ratio (PAR) for each technology in a futuretime slot may be given by:

PAR_(max)=10*log 10(time window/Δt)   (13)

where PAR_(max) is the maximum allowable PAR, time_window is the timeduration of the respective time-averaging window (e.g., the first timewindow 505 for the first technology and the second time window 705 forthe second technology), and Δt is the time duration of the future timeslot. Here, the average transmit power level for each technology in thefuture time slot is given by the respective maximum allowable powerlevel determined for the future time slot.

For example, if the first time window 505 for the first technology is100 seconds and Δt is 5 seconds, then the maximum allowable PAR for thefirst technology equals 20 dB. If the second time window 705 for thesecond technology is 30 seconds and Δt is 5 seconds, then the maximumallowable PAR for the second technology equals 7.8 dB. A problem withthis is that a high PAR may be required for the second technology (e.g.,mmWave communication). For example, for mmWave communication, theaverage transmit power may be approximately 8 dBm and a transmit powerof over 23 dBm may be required to maintain a radio link at a cell edge,which translates into a PAR of over 15 dB.

The maximum PAR for a technology (e.g., the second technology) may beincreased by decreasing Δt (i.e., time duration of one time slot).Decreasing Δt causes the processor 110 to update the maximum allowablepower level for the first technology and the maximum allowable powerlevel for the second technology more frequently, which consumes morecomputational resources for both technologies. However, decreasing Δt inorder to meet a PAR requirement for one technology (e.g., the secondtechnology) may not be needed for another technology (e.g., the firsttechnology) that already meets its PAR requirement. In this case,decreasing Δt (i.e., time duration of one time slot) results in anunnecessary increase in computational resources for the other technology(e.g., first technology).

To address this, aspects of the present disclosure allow the maximumallowable power levels for different technologies to be updated atdifferent rates while still meeting total RF exposure compliance, asdiscussed further below.

In certain aspects, the maximum allowable power level for the secondtechnology is updated in smaller time intervals than the firsttechnology in order to increase the maximum allowable PAR for the secondtechnology without requiring an increase in the computational load forthe first technology. In these aspects, the future time slot 715(q) forthe second technology is further divided (partitioned) into smaller timeslots 1515(1) to 1515(N), as shown in FIG. 15. The processor 110 maydetermine a normalized PD distribution 1520(1) to 1520(N) for each ofthe smaller time slots 1515(1) to 1515(N), as discussed further below.In the description below, the smaller time slots 1515(1) to 1515(N) arereferred to as sub-time slots for ease of discussion.

In these aspects, the processor 110 determines the maximum allowablepower level for the future time slot 715(q) in an outer loop. In theouter loop, the processor 110 determines the maximum allowable powerlevel for the future time slot 715(q) based on the combinedtime-averaged RF exposure distribution 920 as discussed above to ensuretotal RF exposure compliance. The processor 110 updates the maximumallowable power level for the future time slot 715(q) every Δt seconds(i.e., duration of one time slot), as discussed above. For example, ifthe duration of one time slot is 5 seconds, then the processor 110determines the maximum allowable power level for the future time slot715(q) every 5 seconds. Note that the same time slot duration is usedfor the first technology and the second technology in the outer loop tomeet total RF exposure compliance. The operations of the outer loop arediscussed in greater detail above according to certain aspects withreference to FIG. 9.

In an inner loop, the processor 110 determines maximum allowable powerlevels for the sub-time slots 1515(1) to 1515(N) one at a time at a rateof approximately one per At/N seconds, where N is the number of sub-timeslots in the future time slot 715(q) and Δt is the time duration of thefuture time slot 715(q). For example, if N equals 50 and Δt equals 5seconds, then the processor 110 determines a maximum allowable powerlevel for one of the sub-time slots 1515(1) to 1515(N) approximatelyevery 100 milliseconds. Thus, the inner loop increases the update rateby a factor of N. The higher update rate increases the PAR for thesecond technology (e.g., mmWave communication), as discussed above.

For the inner loop, the processor 110 may use the normalized PDdistribution computed for the future time slot 715(q) in the outer loopas a normalized PD limit for the inner loop. For example, in equation(11) discussed above, the normalized PD distribution for the future timeslot 715(q) is given by PD_(norm_q), which may be scaled by the maximumallowable power level determined for the future time slot 715(q).

In the inner loop, the processor 110 may determine a maximum allowablepower level for the n^(th) sub-time slot based on the following:

$\begin{matrix}{{\frac{1}{N}\lbrack {( {\sum_{i = 1}^{i = {n - 1}}{PD}_{norm\_ i}} ) + {PD}_{norm\_ n}} \rbrack} \leq {PD}_{norm\_ lim}} & (14)\end{matrix}$

where N is the total number of sub-time slots 1515(1) to 1515(N), i isan index for the sub-time slots, PD_(norm_i) is the normalized PDdistribution for the i^(th) sub-time slot, PD_(norn_n) is the normalizedPD distribution for the n^(th) sub-time slot, and PD_(nom_lim) is the PDlimit for the inner loop. The PD limit for the inner loop may be givenby the normalized PD distribution computed for the future time slot715(q) in the outer loop, as discussed above.

In equation (14), sub-time slots 1515(1) to 1515(1−n) are past sub-timeslots relative to sub-time slot 1515(n) (i.e., prior in time to sub-timeslot 1515(n)) and therefore correspond to previous transmissions by thesecond transmitter 130. Sub-time slot 1515(n) may be considered a futuresub-time slot relative to sub-time slots 1515(1) to 1515(1−n). Sincesub-time slots 1515(1) to 1515(n−1) correspond to previoustransmissions, the processor 110 can determine the normalized PDdistributions for sub-time slots 1515(1) to 1515(n−1) in equation (14)based on the previous transmission power levels for sub-time slots1515(1) to 1515(n−1). For example, the processor 110 may record thetransmission power levels for sub-time slots 1515(1) to 1515(n−1) anduse the recorded transmission power levels to determine the normalizedPD distributions for sub-time slots 1515(1) to 1515(n−1).

The processor 110 may then determine a transmission power level forsub-time slot 1515(n) such that the expression on the left side ofequation (14) is equal to or less than PD_(norm_lim) (i.e., satisfiesthe condition≤PD_(norn_lim)). For example, the processor 110 maydetermine the condition is met if the peak value in the expression onthe left side is equal to or less than the peak value in PD_(norm_lim).The processor 110 may use then use the determined transmission powerlevel for sub-time slot 1515(n) as a maximum allowable power level forsub-time slot 1515(n). In this regard, the processor 110 may set atransmission power limit for the second transmitter 130 based on themaximum allowable power level for sub-time slot 1515(n) so that thetransmission power level of the second transmitter 130 during sub-timeslot 1515(n) is constrained by the maximum allowable power level forsub-time slot 1515(n).

The processor 110 may determine the maximum allowable power level forsub-time slot 1515(n) before the start of sub-time slot 1515(n) so thatthe maximum allowable power level is ready in time to enforce themaximum allowable power level for sub-time slot 1515(n) (e.g., bysetting the transmission power limit of the second transmitter 130 basedon the maximum allowable power level). Thus, at the time the processor110 determines the maximum allowable power level for sub-time slot1515(n), sub-time slot 1515(n) may be considered a future sub-time slot.

The processor 110 may determine the maximum allowable power level foreach sub-time slot 1515(1) to 1515(N) based on equation (14), in which nis 1 for the first sub-time slot 1515(1) and n is N for the lastsub-time slot 1515(N). In equation (14), the expression on the left sideis an accumulative average given by the combination of the normalized PDdistributions for sub-time slots 1515(1) to 1515(n) divided by N. Forthe example where the time duration of the future time slot 715(q) inthe outer loop is 5 seconds and the time duration of one sub-time slotis 100 milliseconds, N equals 50. Note that the accumulative averageaccumulates more sub-time slots as n increases from 1 for the firstsub-time slot to N for the last sub-time slot (i.e., as the processor110 progresses through the sub-time slots 1515(1) to 1515(N)).

Thus, the processor 110 may perform the above operations in the innerloop to update the maximum allowable power level for the secondtechnology at a higher rate than the outer loop (i.e., update themaximum allowable power level for the second technology every Δt/Nseconds). The processor 110 can accomplish this without having toincrease the update rate for the first technology, and thus withouthaving to increase the computational load for the first technology. Forexample, if the time duration of one time slot in the outer loop is 5seconds and the time duration of one sub-time slot in the inner loop is100 milliseconds, then the maximum allowable power level for the firsttechnology is updated every 5 seconds, and the maximum allowable powerlevel for the second technology is updated every 100 milliseconds. Thus,the processor 110 is able to update maximum allowable power levels forthe first technology and the second technology at different rates (i.e.,different time intervals between updates).

Additional examples for determining maximum allowable power levels forthe sub-time slots 1515(1) to 1515(N) in the inner loop will now bediscussed according to certain aspects of the present disclosure.

FIG. 16 illustrates a time-averaging window 1605 for the inner loopcorresponding to the future time slot 715(q) in the outer loop. Theduration of the time-averaging window 1605 is equal to the duration ofone time slot in the outer loop (labeled “T1”). The time-averagingwindow 1605 may be temporally aligned with the future time slot 715(q)in the outer loop. In one example, the duration T1 of one time slot inthe outer loop is equal to 0.5 seconds (i.e., T1=0.5 sec). Thetime-averaging window 1605 includes N sub-time slots, where the durationof each sub-time slot is equal to T1/N (labeled “T2”). In one example, Nequals 50 and T1 equals 0.5 seconds. In this example, the duration ofone sub-time slot in the inner loop is equal to 10 ms (i.e., T2=10 ms).

In the example in FIG. 16, the inner loop receives a PD limit (labeled“PD_(lim)”) from the outer loop for the time-averaging window 1605. ThePD limit may correspond to the peak PD value in the normalized PDdistribution PD_(norm_lim) determined for the future time slot 715(q)discussed above. In the example in FIG. 16, the PD limit is greater thanone. This is possible because the outer loop determines RF exposurecompliance over a time-averaging window 705 that includes multiple timeslots 715(1)-715(q). This allows the PD limit to exceed one as long asthe average RF exposure over the time-averaging window 705 in the outerloop meets RF exposure compliance. In general, the PD limit from theouter loop may be greater than or less than one.

FIG. 16 shows a point in time in which the maximum allowable power levelis being determined for sub-time slot 1620 within the time-averagingwindow 1605. FIG. 16 shows the PDs (e.g., peak PD values) for thesub-time slots within the time averaging window 1605 preceding sub-timeslot 1620 (referred to as past sub-time slots below). The past sub-timeslots correspond to past transmissions with respect to the point in timeshown in FIG. 16. The PDs for sub-time slots succeeding sub-time slot1620 within the time-averaging window 1605 are not shown in FIG. 16since these sub-time slots correspond to future transmissions withrespect to the point in time shown in FIG. 16.

In this example, the PD limit is met when the PD averaged over thetime-averaging window 1605 is equal to or below the PD limit.Graphically, this condition is met when the area under the PD curve 1610across the entire the time-averaging window 1605 is equal to or lessthan the area under the PD limit line 1630 across the entiretime-averaging window 1605.

In certain aspects, the processor 110 may determine the maximumallowable power level for the ith sub-time slot within thetime-averaging window 1605 based on the following:

PD_(allowed,i) =N·PD_(lim)−(N−i)·PD_(R)Σ_(k=1) ^(k=i−1)PD_(k)   (15)

where PD_(allowed,i) is an allowed PD (e.g., allowed peak PD value) forthe ith sub-time slot within the time-averaging window 1605, theexpression N·PD_(lim) l is the PD allocation for the time-averagingwindow 1605, PDR is a reserved PD, and the summation term sums the PDsof the past sub-time slots within the time-averaging window 1605.Equation (15) can be rewritten into the following equivalent equation:

PD_(allowed,i)=PD_(lim)+(N−i)(PD_(lim)−PD_(R))+Σ_(k=1)^(k=i−1)(PD_(lim)−PD_(k))   (16).

In equation (15), the summation of the PDs of the past sub-time slotsrepresents the portion of the PD allocation for the time-averagingwindow 1605 that has already been used up. The past sub-time slotsinclude the first sub-time slot to the ith-1 sub-time slot in thetime-averaging window 1605. The portion of the PD allocation that hasalready been used up is subtracted from the PD allocation for thetime-averaging window 1605 since the portion of the PD allocation thathas already been used up is not available for the ith sub-time slot.

The term (N-i)·PD_(R) represents the portion of the PD allocation forthe time-averaging window 1605 that is reserved for future sub-timeslots (i.e., sub-time slots within the time-averaging window 1605 afterthe ith sub-time slot). The reserved PD_(R) is multiplied by (N−i),which is the number of future sub-time slots after the ith sub-timeslot. The portion of the PD allocation reserved for future sub-timeslots is subtracted from the PD allocation for the time-averaging window1605 since the portion of the PD allocation reserved for future sub-timeslots is not available for the ith sub-time slot. In certain aspects,the reserved PDR is given by:

PD_(R)=PD_(lim)·PD_(_reserve_ratio)   (17)

where PD_reserve_ratio is a reserve ratio. Thus, the portion of the PDallocation that is reserved for future sub-time slots may be set bysetting the reserve ratio. Examples of factors for setting the reserveratio are discussed further below according to certain aspects.

Thus, when determining the allowed PD for the ith sub-time slot, theprocessor 110 takes into account the portion of the PD allocation forthe time-averaging window 1605 that has already been used up by pastsub-time slots. The processor 110 also reserves a portion of theremaining PD allocation for future sub-time slots. FIG. 17 shows agraphical representation of the portion of the PD allocation that hasalready been used up by past sub-time slots 1710, the allowed PD for theith sub-time slot 1720, and the portion of the remaining PD allocationreserved for future sub-time slots 1730. In FIG. 17, the PD allocationfor the time-averaging window 1605 is represented by the area under thePD limit line 1630.

As discussed above, the processor 110 may set the portion of the PDallocation that is reserved for future sub-time slots by setting thereserve ratio. In one example, the processor 110 may set the reserveratio based on channel conditions between the wireless device 100 (e.g.,mobile wireless device) and another wireless device (e.g., base station)receiving transmissions from the wireless device 100. For example, ifchannel conditions are good (e.g., low interference and/or short rangebetween the wireless devices), then the processor 110 may set thereserve ratio higher to more evenly spread out transmission power acrossthe time-averaging window 1605. In this case, spreading out thetransmission power may result in higher throughput. Setting the reserveratio higher more evenly spreads out the transmission power by reservinga larger portion of the PD allocation for future sub-time slots.

If channel conditions are bad (e.g., high interference and/or long rangebetween the wireless devices), then the processor 110 may set thereserve ratio lower. Setting the reserve ratio lower reduces the portionof the PD allocation reserved for future sub-time slots. This relaxesthe constraint on the transmission power for a current sub-time slotimposed by the portion of the PD allocation reserved for future sub-timeslots, allowing the transmitter 130 to transmit at a higher power forthe current sub-time slot. In this case, transmitting at higher power(e.g., in a short burst) may be necessary to ensure that datatransmitted from the wireless device 100 is successfully received by theother wireless device.

In another example, the processor 110 may set the reserve ratio based ondata traffic patterns. For example, if the wireless device 100 isscheduled to transmit data toward the end of the time-averaging window1605, then the processor 110 may set the reserve ratio higher to reservea sufficient amount of the PD allocation for the scheduled datatransmission. This may help prevent the transmitter 130 from using upall or most of the PD allocation before the scheduled data transmission,and not leaving enough of the PD allocation for the scheduled datatransmission. In another example, if the wireless device 100 isscheduled to transmit data for a short duration toward the beginning ofthe time-averaging window 1605 with no other scheduled data transmissionfor the rest of the time-averaging window 1605, then the processor 110may set the reserve ratio lower since reserving a portion of the PDallocation for another data transmission may not be necessary in thisexample. In another example, the processor 110 may predict futuretransmissions and/or frequencies based on past transmissions and/orupper layer information. The processor 110 may then set the reserveratio based on the predicted future transmissions and/or frequencies.For example, if data packets (e.g., data packets of a certain size) aretransmitted at certain time intervals in the past, then the processor110 may use this information to predict that data packets will betransmitted at the time intervals in the future, and set the reserveratio accordingly (e.g., set the reserve ratio to provide enough reservefor the predicted data packet transmissions within the time-averagingwindow 1605).

The processor 110 may determine an allowed PD for each of the sub-timeslots within the time-averaging window 1605 for the inner loop (e.g.,based on equation (15)). For the first sub-time slot, the portion of thePD allocation that has been used up is zero since there are no sub-timeslots within the time-averaging window 1605 preceding the first sub-timeslot. For the last sub-time slot, the portion of the PD allocationreserved for future sub-time slots is zero since there are no sub-timeslots within the time-averaging window 1605 after the last sub-timeslot. After determining the allowed PD for each sub-time slot, theprocessor 110 may determine a maximum allowable power level for thesub-time slot, and set a transmission power limit of the transmitter 130for the sub-time slot based on the determined maximum allowable powerlevel.

In certain aspects, the processor 110 may determine the maximumallowable power level for a sub-time slot based on the allowed PD forthe sub-time slot using a table. In this regard, FIG. 18 shows anexample of a table 1810 for converting an allowed PD for a sub-time slotinto a maximum allowable power level. The table 1810 may be stored inthe memory 115.

In this example, the table 1810 includes an index for n differenttransmit scenarios for the transmitter 130. The n transmit scenarios maycorrespond to different beams, different user positions relative to thewireless device 100, etc. For each transmit scenario in the table 1810,the table 1810 includes a corresponding transmission power level at areference PD (labeled “Tx₁” to “Tx_(n)”). The transmission power levelsfor the different transmit scenarios at the reference PD may bepredetermined by performing simulations and/or measurements on thewireless device 110, and prestored in the table 1810.

To determine the maximum allowable power level for a sub-time slot basedon the corresponding allowed PD, the processor 110 may determine thetransmit scenario for the sub-time slot and retrieve the transmissionpower level in the table 1810 corresponding to the determined transmitscenario. For example, if the transmit scenarios in table 1810correspond to different beams, then the processor 110 may determine thebeam for the sub-time slot, and retrieve the transmission power level inthe table 1810 corresponding to the determined beam.

After retrieving the transmission power level from the table 1810, theprocessor 110 may determine the maximum allowable power level for thesub-time slot by scaling the transmission power level from the table1810 based on the allowed PD for the sub-time slot and the reference PD(e.g., scale the transmission power level from the table 1810 by a ratioof the allowed PD to the reference PD). For example, if the allowed PDis 50% smaller than the reference PD, then the processor 110 may reducethe transmission power level from the table 1810 by 50% to obtain themaximum allowable power level for the sub-time slot. If the transmissionpower level in the table 1810 is in decibels (dB), then the maximumallowable power level is obtained in this example by subtracting 3 dBfrom the transmission power in the table 1810. Thus, the maximumallowable power level for any allowed PD may be obtain by scaling thetransmission power level in the table 1810 based on the allowed PD andthe reference PD. This is possible since PD scales with transmissionpower, as discussed above.

As discussed above, when determining the allowed PD for a sub-time slot,the processor 110 may take into account the portion of the PD allocationthat has been used up by past sub-time slots. In the example in equation(15), the portion of the PD allocation that has been used up by the pastsub-time slots is determined by summing the PDs of the past sub-timeslots. In one example, the PDs of the past sub-time slots may correspondto the allowed PDs determined for the past sub-time slots. In anotherexample, the PDs of the past sub-time slots may correspond to the actualPDs for the past sub-time slots. The actual PD for a past sub-time slotmay be the PD corresponding to the actual transmission power level usedduring the past sub-time slot, which may be equal to or less than themaximum allowable power level determined for the past sub-time slot. Inthis example, the processor 110 may record the actual transmission powerlevels used for the past sub-time slots, and determine the actual PDsfor the past sub-time slots based on the actual transmission powerlevels for the past sub-time slots.

It is to be appreciated that the present disclosure is not limited toone inner loop. For example, the processor 110 may determine maximumallowable power levels using two inner loops including a first innerloop and a second inner loop. In this example, the inner loop discussedabove may correspond to the first inner loop, which receives the PDlimit for the future time slot 715(q) from the outer loop, and adjuststhe PD within the future time slot in sub-time slots such that the PDlimit from the outer loop is met when the PD is averaged over the futuretime slot.

In this example, the second inner loop is below the first inner loop.The second inner loop partitions each sub-time slot in the first innerloop into smaller sub-time slots. For each sub-time slot of the firstinner loop, the second inner loop receives a PD limit from the firstinner loop (e.g., allowed PD for the sub-time slot of the inner loop),and adjusts the maximum allowable power level within the sub-time slotsuch that the PD limit is met. Within a sub-time slot of the first innerloop, the second inner loop determines the maximum allowable power levelfor each sub-time slot of the second inner loop within the sub-time slotof the first inner loop (e.g., based on equation (15) in which the PDlimit is provided by the first inner loop). In this example, the maximumallowable power level is determined for each sub-time slot of the secondinner loop.

In certain aspects, the processor 110 may use any of the techniquesdiscussed above to determine maximum allowable power levels fordifferent sub-6 GHz technologies at different rates. For example, theprocessor 110 may update the maximum allowable power level for LTE every100 milliseconds, but update the maximum allowable power level for GSMevery 5 seconds depending on how frequently or infrequently a technologychanges. In this example, the processor 110 may determine a maximumallowable power level for LTE and a maximum allowable power level forGSM every 5 seconds in an outer loop. For LTE, the processor 110 mayfurther divide a future time slot in the outer loop into sub-time slots(e.g., 50 sub-time slots), and determine a maximum allowable power levelfor each sub-time slot in an inner loop. The processor 110 may use thenormalize SAR distribution for the future time slot in the outer loop asa normalized SAR limit in the inner loop for determining the maximumallowable power level for each sub-time slot in the inner loop.

In certain aspects, the techniques discussed above may be extended torunning more than two different time intervals (i.e., update maximumallowable power levels for different technologies at more than twodifferent update rates). To do this, the processor 110 may run differentinner loops for different technologies in which the duration of onesub-time slot in each inner loop is different. In this example, theduration of one sub-time slot for each inner loop may be chosen based onthe desired time interval (i.e., update rate) for the correspondingtechnology. In some cases, each technology may run at a specific timeinterval that is different from other technologies.

As discussed above, the processor 110 may determine a maximum allowablepower level for a technology for a future time slot and set atransmission power limit of the corresponding transmitter (e.g.,transmitter 120 or 130) based on the determined maximum allowable powerlevel. The future time slot may be a future time slot in an outer loop(e.g., 5-second future time slot) or a future sub-time slot in an innerloop (e.g., e.g., 100-millisecond future sub-time slot). In certainaspects, the maximum allowable power level discussed above is used as amaximum allowable time-average power level for the future time slot, andtherefore may be considered a maximum allowable time-average power levelfor the future time slot in these aspects. In the discussion below, themaximum allowable power level discussed above is referred to as themaximum allowable time-average power level.

In the above aspects, setting the transmission power limit of atransmitter based on the determined maximum allowable time-average powerlevel prevents a time-average of the transmission power level of thetransmitter over the future time slot from exceeding the maximumallowable time-average power level. The instantaneous transmission powerlevel of the transmitter is allowed to exceed the maximum allowabletime-average power level during the future time slot as long as thetime-average of the power level over the future time slot does notexceed the maximum allowable time-average power level.

In the above example, the maximum allowable time-average power level fora future time slot is used to set the maximum power limit of thecorresponding transmitter so that the time-average of the transmissionpower level over the future time slot does not exceed the maximumallowable time-average power level.

In certain aspects, the processor 110 may convert the maximum allowabletime-average power level for a future time slot into a maximum allowableduty cycle for the future time slot as follows:

Max_duty_cycle=Max_avg_P/P _(max)   (18)

where Max_duty_cycle is the maximum allowable duty cycle for the futuretime slot, Max_avg_P is the maximum allowable time-average power levelfor the future time slot, and P_(max) is a maximum allowable powerlevel. The maximum allowable power P_(max) may be a limit on the maximumallowable instantaneous power level or peak power level of thecorresponding transmitter. The maximum allowable power P_(max) may bedefined by a standard or a regulator body. In this example, thetransmitter may transmit at a power level approximately equal to P_(max)during the future time slot at a duty cycle approximately equal to themaximum allowable duty cycle. The maximum duty cycle may have a valuebetween 0 to 1.

In certain aspects, after determining the maximum allowable time-averagepower level for a future time slot (e.g., using any of the methoddiscussed above), the processor 110 may determine a correspondingmaximum allowable duty cycle for the future time slot based on equation(18). The processor 110 may then set a transmission duty cycle limit ofthe corresponding transmitter (e.g., transmitter 120 or 130) for thefuture time slot based on the determined the maximum allowable dutycycle. In this example, setting the transmission duty cycle limit basedon the determined the maximum allowable duty cycle prevents thetransmission duty cycle of the transmitter from exceeding the maximumallowable duty cycle for the future time slot. In other words, thetransmission duty cycle of the transmitter for the future time slot isconstrained by the determined maximum allowable duty cycle.

In certain aspects, the processor 110 may convert a maximum allowabletime-average power level for a future time slot into a maximum allowablepeak power for the future time slot as follows:

Max_peak_power=Max_avg_P/Max_duty_cycle   (19)

where Max_peak_power is the maximum allowable peak power for the futuretime slot, Max_duty_cycle is a maximum allowable duty cycle for thefuture time slot, and Max_avg_P is the maximum allowable time-averagepower level for the future time slot. The maximum allowable duty cyclecan be provided by a radio frequency (RF) system (e.g., through uplinkgrant provided by the network), defined by standard, etc.

In certain aspects, after determining the maximum allowable time-averagepower level for a future time slot (e.g., using any of the methoddiscussed above), the processor 110 may determine a correspondingmaximum peak power for the future time slot based on equation (19). Theprocessor 110 may then set a peak power limit of the correspondingtransmitter (e.g., transmitter 120 or 130) for the future time slotbased on the determined maximum allowable peak power. In this example,setting the peak power limit of the transmitter for the future time slotbased on the determined the maximum allowable peak power prevents thepeak power of the transmitter from exceeding the maximum allowable peakpower. In other words, the instantaneous power level of the transmitterduring the future time slot is constrained by the maximum allowable peakpower so that the peak power during the future time slot does not exceedthe maximum allowable peak power.

The processor 110 may also generate a table including differentcombinations of maximum allowable duty cycle and maximum allowable peakpower for the future time slot based on the maximum allowabletime-average power level for the future time slot. For example, theprocessor 110 may generate the table by inputting different maximumallowable duty cycles into equation (19) and determining a maximumallowable peak power for each of the different maximum allowable dutycycles. In this example, the processor may dynamically adjust the dutycycle based on the peak power during the future time slot using thetable or adjust the peak power based on the transmission duty cycleduring the future time slot using the table.

Thus, the processor 110 may convert the maximum allowable time-averagepower level for a future time slot into a maximum allowable duty cyclefor the future time slot and set a transmission duty cycle limit of thecorresponding transmitter based on the maximum allowable duty cycle. Theprocessor 110 may also convert the maximum allowable time-average powerlevel for a future time slot into a maximum allowable peak power for thefuture time slot and set a peak power limit of the correspondingtransmitter based on the maximum allowable peak power. The maximumallowable time-average power level for the future time slot may bedetermined using any of the methods discussed above. For the example inwhich the future time slot is a future time slot in an outer loop, themaximum allowable time-average power level may correspond to the maximumallowable power level computed for the future time slot. For the examplein which the future time slot is a future sub-time slot in an innerloop, the maximum allowable time-average power level may correspond tothe maximum allowable power level computed for the future sub-time slot(e.g., 100-millisecond sub-time slot). It is to be appreciated that thepresent disclosure in not limited to these examples, and that themaximum allowable time-average power level for a future time slot may bedetermined using other methods.

For example, a transmitter (e.g., transmitter 120 and 130) may transmitover a time window divided (discretized) into N time slots (not to beconfused with the number of sub-time slots in FIG. 18). In this example,the maximum allowable time-average power level for a future time slotmay be determined by the following:

$\begin{matrix}{{\frac{1}{N}\lbrack {( {\sum_{k = 1}^{N - 1}\frac{{avg}\; P_{tk}}{P_{limit}}} ) + \frac{{Max\_ avg}\; P_{tN}}{P_{limit}}} \rbrack} \leq 1} & (20)\end{matrix}$

where P_(limit) is a time-average transmission power which results in anRF exposure corresponding to regulator SAR limit or PD limit, avgP_(tk)is the average transmission power in the k_(th) time slot, the N^(th)time slot is the future time slot, and Max_avgP_(tN) is the maximumallowable time-average power level for the future time slot. In equation(20), the average transmission power level is normalized with respect toP_(limit.) The time window may be a moving time-averaging window, inwhich the maximum allowable time-average power level for the future timeslot is updated every time_window/N seconds where time_window is theduration of the time window. In this example, the first time slot to thek_(th) time slot in equation (20) correspond to previous transmissionsby the transmitter. Thus, the average transmission power levels forthese time slots are known. In this example, the processor 110 maydetermine the maximum allowable time-average power level Max_avgP_(tk)for the future time slot by determining a maximum allowable time-averagepower level Max_avgP_(tk) satisfying the condition in equation (20). Theprocessor 110 may do this by determining a maximum allowabletime-average power level Max_avgP_(tk) that results in the normalizedtime average power on the left side of equation (20) being equal to orless than one.

In the above example, the processor 110 may convert the maximumallowable time-average power level Max_avgP_(tk) for the future timeslot into a maximum allowable duty cycle for the future time slot (e.g.,based on equation (18) using Max_avgP_(tk) for Max_avg_P in equation(18)), and set a transmission duty cycle limit of the correspondingtransmitter based on the maximum allowable duty cycle. The processor 110may also convert the maximum allowable time-average power levelMax_avgP_(tk) for the future time slot into a maximum allowable peakpower for the future time slot (e.g., based on equation (19) usingMax_avgP_(tk) for Max_avg_P in equation (19)), and set a peak powerlimit of the corresponding transmitter based on the maximum allowablepeak power.

In certain aspects, the memory 115 may include a computer readablemedium including instructions stored thereon that, when executed by theprocessor 110, cause the processor 110 to perform the method 1400 and/orany other method described herein. The computer readable medium mayinclude, by way of example, RAM (Random Access Memory), flash memory,ROM (Read Only Memory), PROM (Programmable Read-Only Memory), EPROM(Erasable Programmable Read-Only Memory), EEPROM (Electrically ErasableProgrammable Read-Only Memory), registers, magnetic disks, opticaldisks, hard drives, or any other tangible non-transitory storage medium,or any combination thereof.

In certain aspects, an apparatus may include means for performing themethod 1400. The apparatus may include means for determining a specificabsorption rate (SAR) distribution for a first wireless communicationtechnology, means for determining a power density (PD) distribution fora second wireless communication technology, and means for combining theSAR distribution and the PD distribution to generate a combined RFexposure distribution. The apparatus may also include means fordetermining at least one first maximum allowable power level and atleast one second maximum allowable power level for a future time slotbased on the combined RF exposure distribution, means for setting atleast one transmission power limit for a first transmitter in the futuretime slot based on the at least one first maximum allowable power level,and means for setting at least one transmission power limit for a secondtransmitter in the future time slot based on the at least one secondmaximum allowable power level.

It is to be appreciated that the present disclosure is not limited tothe exemplary terms used above to describe aspects of the presentdisclosure, and that the present disclosure covers equivalent terms. Forexample, it is to be appreciated that a distribution may also bereferred to as a map, a scan, or another term. In another example, it isto be appreciated that an antenna may also be referred to as an antennaelement or another term. In yet another example it is to be appreciatedthat a maximum allowable power level may also be referred to as a powerlevel limit or another term.

The term “approximately”, as used herein with respect to a stated valueor a property, is intended to indicate being within 10% of the statedvalue or property.

Any reference to an element herein using a designation such as “first,”“second,” and so forth does not generally limit the quantity or order ofthose elements. Rather, these designations are used herein as aconvenient way of distinguishing between two or more elements orinstances of an element. Thus, a reference to first and second elementsdoes not mean that only two elements can be employed, or that the firstelement must precede the second element.

Within the present disclosure, the word “exemplary” is used to mean“serving as an example, instance, or illustration.” Any implementationor aspect described herein as “exemplary” is not necessarily to beconstrued as preferred or advantageous over other aspects of thedisclosure. Likewise, the term “aspects” does not require that allaspects of the disclosure include the discussed feature, advantage ormode of operation.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A wireless device, comprising: a transmitter; anda processor coupled to the transmitter, wherein the processor isconfigured to: receive a power density (PD) limit for a time slot, thetime slot comprising sub-time slots; determine a PD allocation for thetime slot based on the PD limit; determine a maximum allowable powerlevel for one of the sub-time slots based on a portion of the PDallocation used up by one or more of the sub-time slots preceding theone of the sub-time slots; and set a transmission power limit for thetransmitter based on the maximum allowable power level.
 2. The wirelessdevice of claim 1, wherein, during the one of the sub-time slots, thetransmitter is configured to transmit a signal at a transmission powerlevel equal to or less than the transmission power limit.
 3. Thewireless device of claim 1, wherein the determination by the processorof the PD allocation for the time slot comprises multiplying the PDlimit by a number of the sub-time slots.
 4. The wireless device of claim1, wherein the processor is configured to determine the PD allocationused up by the one or more of the sub-time slots preceding the one ofthe sub-time slots.
 5. The wireless device of claim 4, wherein thedetermination by the processor of the PD allocation used up by the oneor more of the sub-time slots preceding the one of the sub-time slotscomprises summing PDs of the one or more of the sub-time slots precedingthe one of the sub-time slots.
 6. The wireless device of claim 1,wherein the processor is further configured to: determine a portion ofthe PD allocation reserved for one or more of the sub-time slotssucceeding the one of the sub-time slots; and determine the maximumallowable power level for the one of the sub-time slots based also onthe portion of the PD allocation reserved for the one or more of thesub-time slots succeeding the one of the sub-time slots.
 7. The wirelessdevice of claim 6, wherein the determination by the processor of theportion of the PD allocation reserved for the one or more of thesub-time slots succeeding the one of the sub-time slots comprisesmultiplying a reserved PD by a number of the one or more of the sub-timeslots succeeding the one of the sub-time slots.
 8. The wireless deviceof claim 7, wherein the processor is configured to determine thereserved PD based on the PD limit and a reserve ratio.
 9. The wirelessdevice of claim 8, wherein the determination by the processor of thereserved PD comprises multiplying the PD limit by the reserve ratio. 10.The wireless device of claim 8, wherein the processor is configured toset the reserve ratio based on at least one of a channel conditionbetween the wireless device and another wireless device, or a scheduleddata transmission for the wireless device.
 11. The wireless device ofclaim 1, wherein the determination by the processor of the maximumallowable power level for the one of the sub-time slots comprises:determining an allowed PD for the one of the sub-time slots based on theportion of the PD allocation used up by the one or more of the sub-timeslots preceding the one of the sub-time slots; retrieving a transmissionpower level from a memory, the transmission power level corresponding toa reference PD; and determining the maximum allowable power level forthe one of the sub-time slots based on the retrieved transmission powerlevel, the allowed PD, and the reference PD.
 12. The wireless device ofclaim 11, wherein the determination by the processor of the maximumallowable power level for the one of the sub-time slots based on theretrieved transmission power level, the allowed PD, and the reference PDcomprises scaling the retrieved transmission power level by a ratio ofthe allowed PD to the reference PD.
 13. The wireless device of claim 1,wherein the processor is configured to: determine a time-averaged radiofrequency (RF) exposure distribution; and determine the PD limit for thetime slot based on the time-averaged RF exposure distribution.
 14. Thewireless device of claim 13, wherein the determination of thetime-averaged RF exposure distribution by the processor comprises:determining a time-averaged specific absorption (SAR) distribution overa first time window; determining a time-averaged power density (PD)distribution over a second time window; and combining the time-averagedSAR distribution and the time-averaged PD distribution to generate thetime-averaged RF exposure distribution.
 15. The wireless device of claim14, wherein the first time window and the second time window havedifferent lengths.
 16. The wireless device of claim 13, wherein thetime-averaged RF exposure distribution comprises a time-averaged powerdensity (PD) distribution.
 17. A method implemented in a wirelessdevice, comprising: receiving a power density (PD) limit for a timeslot, the time slot comprising sub-time slots; determining a PDallocation for the time slot based on the PD limit; determining amaximum allowable power level for one of the sub-time slots based on aportion of the PD allocation used up by one or more of the sub-timeslots preceding the one of the sub-time slots; and setting atransmission power limit for a transmitter based on the maximumallowable power level.
 18. The method of claim 17, further comprising:determining a time-averaged radio frequency (RF) exposure distribution;and determining the PD limit for the time slot based on thetime-averaged RF exposure distribution.
 19. The method of claim 18,wherein determining the time-averaged RF exposure distributioncomprises: determining a time-averaged specific absorption (SAR)distribution over a first time window; determining a time-averaged powerdensity (PD) distribution over a second time window; and combining thetime-averaged SAR distribution and the time-averaged PD distribution togenerate the time-averaged RF exposure distribution.
 20. A wirelessdevice, comprising: a transmitter; and a processor coupled to thetransmitter, wherein the processor is configured to: determine atime-averaged radio frequency (RF) exposure distribution over timeslots; determine an RF exposure limit for one of the time slots based onthe time-averaged RF exposure distribution over the time slots, the oneof the time slots comprising sub-time slots; determine a time-averagedRF exposure distribution within the one of the time slots; determine amaximum allowable power level for one of the sub-time slots based on theRF exposure limit and the time-averaged RF exposure distribution withinthe one of the time slots; and set a transmission power limit for thetransmitter based on the maximum allowable power level.
 21. The wirelessdevice of claim 20, wherein the determination of the time-averaged RFexposure distribution over the time slots by the processor comprises:determining a time-averaged specific absorption (SAR) distribution overa first time window; determining a time-averaged power density (PD)distribution over a second time window; and combining the time-averagedSAR distribution and the time-averaged PD distribution to generate thetime-averaged RF exposure distribution over the time slots.
 22. Thewireless device of claim 21, wherein the first time window and thesecond time window have different lengths.
 23. The wireless device ofclaim 21, wherein the time-averaged SAR distribution corresponds to afirst technology and the time-averaged PD distribution corresponds to asecond technology.
 24. The wireless device of claim 23, wherein thefirst technology comprises a sub-6 GHz technology and the secondtechnology comprises a millimeter wave technology.
 25. The wirelessdevice of claim 23, wherein a same time-slot duration is used for thefirst technology and the second technology.
 26. The wireless device ofclaim 21, wherein the time-averaged SAR distribution is normalized withrespect to a SAR limit, and the time-averaged PD distribution isnormalized with respect to a PD limit.
 27. The wireless device of claim21, wherein the RF exposure limit comprises a PD limit.
 28. The wirelessdevice of claim 20, wherein the time-averaged RF exposure distributionover the time slots comprises a time-averaged power density (PD)distribution, and the RF exposure limit comprises a PD limit.
 29. Thewireless device of claim 28, wherein the sub-time slots are normalizedwith respect to the PD limit.
 30. The wireless device of claim 20,wherein the time-averaged RF exposure distribution over the time slotscomprises a time-averaged specific absorption rate (SAR) distribution,and the RF exposure limit comprises a SAR limit.
 31. The wireless deviceof claim 30, wherein the sub-time slots are normalized with respect tothe SAR limit.
 32. The wireless device of claim 20, wherein the one ofthe sub-time slots comprises second sub-time slots, and the processor isconfigured to: determine a second RF exposure limit for the one of thesub-time slots; determine a time-averaged RF exposure distributionwithin the one of the sub-time slots; and determine a maximum allowablepower level for one of the second sub-time slots based on the second RFexposure limit and the time-averaged RF exposure distribution within theone of the sub-time slots.
 33. The wireless device of claim 20, whereinthe processor is configured to: determine a second RF exposure limit fora second one of the time slots based on the time-averaged RF exposuredistribution over the time slots, the second one of the time slotscomprising second sub-time slots; determine a time-averaged RF exposuredistribution within the second one of the time slots; and determine amaximum allowable power level for one of the second sub-time slots basedon the second RF exposure limit and the time-averaged RF exposuredistribution within the second one of the time slots.
 34. The wirelessdevice of claim 33, wherein the first sub-time slots and the secondsub-time slots have different time intervals.
 35. The wireless device ofclaim 20, wherein the RF exposure limit comprises a power density (PD)limit, and the processor is configured to: determine a PD allocationbased on the PD limit; determine a portion of the PD allocation reservedfor one or more of the sub-time slots succeeding the one of the sub-timeslots; and determine the maximum allowable power level for the one ofthe sub-time slots based also on the portion of the PD allocationreserved for the one or more of the sub-time slots succeeding the one ofthe sub-time slots.
 36. The wireless device of claim 35, wherein thetime-averaged RF exposure distribution within the one of the time slotsis averaged over one or more of the sub-time slots preceding the one ofthe time slots.
 37. The wireless device of claim 35, wherein thedetermination by the processor of the portion of the PD allocationreserved for the one or more of the sub-time slots succeeding the one ofthe sub-time slots comprises multiplying a reserved PD by a number ofthe one or more of the sub-time slots succeeding the one of the sub-timeslots.
 38. The wireless device of claim 37, wherein the processor isconfigured to determine the reserved PD based on the PD limit and areserve ratio.
 39. The wireless device of claim 38, wherein thedetermination by the processor of the reserved PD comprises multiplyingthe PD limit by the reserve ratio.
 40. The wireless device of claim 38,wherein the processor is configured to set the reserve ratio based on atleast one of a channel condition between the wireless device and anotherwireless device, or a scheduled data transmission for the wirelessdevice.